What does enctype='multipart/form-data' mean?

Asked 24/12, 2010 at 12:19 Answered 11/11, 2018 at 22:52

Solved html http-headers multipartform-data

1893

What does enctype='multipart/form-data' mean in an HTML form and when should we use it?

Lyricist answered 24/12, 2010 at 12:19 Comment(3)

w3.org/html/wg/spec/… – Rosefish 9/8, 2018 at 10:26

It's for POSTing entire files as part of form submission. I thought it might have something to do with multiple parts of text-form input, but it doesn't, it's just for file uploads. – Castilian 7/5, 2021 at 0:6

In form submission, enc in enctype attribute is short form for ENCoding. The RFC7578 is here, the obsoleted RFC2388 here and the algorithm here – Saguache 19/12, 2023 at 1:58

2043

When you make a POST request, you have to encode the data that forms the body of the request in some way.

HTML forms provide three methods of encoding.

application/x-www-form-urlencoded (the default)
multipart/form-data
text/plain

Work was being done on adding application/json, but that has been abandoned.

(Other encodings are possible with HTTP requests generated using other means than an HTML form submission. JSON is a common format for use with web services and some still use SOAP.)

The specifics of the formats don't matter to most developers. The important points are:

Never use text/plain.

When you are writing client-side code:

use multipart/form-data when your form includes any <input type="file"> elements
otherwise you can use multipart/form-data or application/x-www-form-urlencoded but application/x-www-form-urlencoded will be more efficient

When you are writing server-side code:

Use a prewritten form handling library

Most (such as Perl's CGI->param or the one exposed by PHP's $_POST superglobal) will take care of the differences for you. Don't bother trying to parse the raw input received by the server.

Sometimes you will find a library that can't handle both formats. Node.js's most popular library for handling form data is body-parser which cannot handle multipart requests (but has documentation that recommends some alternatives which can).

If you are writing (or debugging) a library for parsing or generating the raw data, then you need to start worrying about the format. You might also want to know about it for interest's sake.

application/x-www-form-urlencoded is more or less the same as a query string on the end of the URL.

multipart/form-data is significantly more complicated but it allows entire files to be included in the data. An example of the result can be found in the HTML 4 specification.

text/plain is introduced by HTML 5 and is useful only for debugging — from the spec: They are not reliably interpretable by computer — and I'd argue that the others combined with tools (like the Network Panel in the developer tools of most browsers) are better for that).

Holography answered 24/12, 2010 at 12:21 Comment(9)

@Holography Excuse me, what will be any probable problem if we use multipart for all forms? with and whit out files. – Kealey 20/10, 2013 at 9:47

It doesn't make sense for GET forms, and it makes the file size of requests bigger. – Holography 20/10, 2013 at 10:46

@Holography does multipart form data send as a stream by default? – Shirl 13/2, 2017 at 23:6

Does the enc in enctype stand for something? – Nolan 10/12, 2017 at 21:54

"HTML forms provide three methods of ENCoding" – Holography 10/12, 2017 at 22:15

@Holography - why is it called "multipart" ? – Demean 1/9, 2020 at 22:37

Because it has multiple parts – Holography 1/9, 2020 at 23:14

@Demean because it can have multiple data items separated by boundaries, see RFC 2046 section 5.1.1. – Zohara 2/2, 2021 at 14:27

Unless I misunderstand something, application/json is supported now, and has been for some time... – Margarita 11/12, 2023 at 13:3

643

when should we use it?

Quentin's answer is right: use multipart/form-data if the form contains a file upload, and application/x-www-form-urlencoded otherwise, which is the default if you omit enctype.

I'm going to:

add some more HTML5 references
explain why he is right with a form submit example

HTML5 references

There are three possibilities for enctype:

application/x-www-form-urlencoded
multipart/form-data (spec points to RFC7578)
text/plain. This is "not reliably interpretable by computer", so it should never be used in production, and we will not look further into it.

How to generate the examples

Once you see an example of each method, it becomes obvious how they work, and when you should use each one.

You can produce examples using:

nc -l or an ECHO server: HTTP test server accepting GET/POST requests
a user agent like a browser or cURL

Save the form to a minimal .html file:

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8"/>
  <title>upload</title>
</head>
<body>
<form action="http://localhost:8000" method="post" enctype="multipart/form-data">
  <p><input type="text" name="text1" value="text default">
  <p><input type="text" name="text2" value="a&#x03C9;b">
  <p><input type="file" name="file1">
  <p><input type="file" name="file2">
  <p><input type="file" name="file3">
  <p><button type="submit">Submit</button>
</form>
</body>
</html>

We set the default text value to aωb, which means aωb because ω is U+03C9, which are the bytes 61 CF 89 62 in UTF-8.

Create files to upload:

echo 'Content of a.txt.' > a.txt

echo '<!DOCTYPE html><title>Content of a.html.</title>' > a.html

# Binary file containing 4 bytes: 'a', 1, 2 and 'b'.
printf 'a\xCF\x89b' > binary

Run our little echo server:

while true; do printf '' | nc -l localhost 8000; done

Open the HTML on your browser, select the files and click on submit and check the terminal.

nc prints the request received.

Tested on: Ubuntu 14.04.3, nc BSD 1.105, Firefox 40.

multipart/form-data

Firefox sent:

POST / HTTP/1.1
[[ Less interesting headers ... ]]
Content-Type: multipart/form-data; boundary=---------------------------735323031399963166993862150
Content-Length: 834

-----------------------------735323031399963166993862150
Content-Disposition: form-data; name="text1"

text default
-----------------------------735323031399963166993862150
Content-Disposition: form-data; name="text2"

aωb
-----------------------------735323031399963166993862150
Content-Disposition: form-data; name="file1"; filename="a.txt"
Content-Type: text/plain

Content of a.txt.

-----------------------------735323031399963166993862150
Content-Disposition: form-data; name="file2"; filename="a.html"
Content-Type: text/html

<!DOCTYPE html><title>Content of a.html.</title>

-----------------------------735323031399963166993862150
Content-Disposition: form-data; name="file3"; filename="binary"
Content-Type: application/octet-stream

aωb
-----------------------------735323031399963166993862150--

For the binary file and text field, the bytes 61 CF 89 62 (aωb in UTF-8) are sent literally. You could verify that with nc -l localhost 8000 | hd, which says that the bytes:

61 CF 89 62

were sent (61 == 'a' and 62 == 'b').

Therefore it is clear that:

Content-Type: multipart/form-data; boundary=---------------------------735323031399963166993862150 sets the content type to multipart/form-data and says that the fields are separated by the given boundary string.

But note that the:
```
boundary=---------------------------735323031399963166993862150
```
has two less dashes -- than the actual barrier
```
-----------------------------735323031399963166993862150
```
This is because the standard requires the boundary to start with two dashes --. The other dashes appear to be just how Firefox chose to implement the arbitrary boundary. RFC 7578 clearly mentions that those two leading dashes -- are required:

4.1. "Boundary" Parameter of multipart/form-data

As with other multipart types, the parts are delimited with a boundary delimiter, constructed using CRLF, "--", and the value of the "boundary" parameter.
every field gets some sub headers before its data: Content-Disposition: form-data;, the field name, the filename, followed by the data.

The server reads the data until the next boundary string. The browser must choose a boundary that will not appear in any of the fields, so this is why the boundary may vary between requests.

Because we have the unique boundary, no encoding of the data is necessary: binary data is sent as is.

TODO: what is the optimal boundary size (log(N) I bet), and name / running time of the algorithm that finds it? Asked at: https://cs.stackexchange.com/questions/39687/find-the-shortest-sequence-that-is-not-a-sub-sequence-of-a-set-of-sequences
Content-Type is automatically determined by the browser.

How it is determined exactly was asked at: How is mime type of an uploaded file determined by browser?

application/x-www-form-urlencoded

Now change the enctype to application/x-www-form-urlencoded, reload the browser, and resubmit.

Firefox sent:

POST / HTTP/1.1
[[ Less interesting headers ... ]]
Content-Type: application/x-www-form-urlencoded
Content-Length: 51

text1=text+default&text2=a%CF%89b&file1=a.txt&file2=a.html&file3=binary

Clearly the file data was not sent, only the basenames. So this cannot be used for files.

As for the text field, we see that usual printable characters like a and b were sent in one byte, while non-printable ones like 0xCF and 0x89 took up 3 bytes each: %CF%89!

Comparison

File uploads often contain lots of non-printable characters (e.g. images), while text forms almost never do.

From the examples we have seen that:

multipart/form-data: adds a few bytes of boundary overhead to the message, and must spend some time calculating it, but sends each byte in one byte.
application/x-www-form-urlencoded: has a single byte boundary per field (&), but adds a linear overhead factor of 3x for every non-printable character.

Therefore, even if we could send files with application/x-www-form-urlencoded, we wouldn't want to, because it is so inefficient.

But for printable characters found in text fields, it does not matter and generates less overhead, so we just use it.

Godfrey answered 7/2, 2015 at 9:52 Comment(23)

Great post. Question: Why do we have 3 bytes for each non printable character? For eg: for unicode U+03C9, we have %CF%89: this is 2 extra bytes for the two "%". Is my understanding correct? – Huffish 6/8, 2015 at 18:42

@Khanna111%CF is 3 bytes long: %, C and F :-) Story of making it human readable. – Godfrey 6/8, 2015 at 18:50

I had to use a slightly different listener: while true; do printf '' | nc -lp 8000; done – Fricke 27/8, 2015 at 18:36

@Fricke what error did you get on the command I've given? What is your nc version and distro? I'm netcat-openbsd 1.105-7ubuntu1 Ubuntu 14.04.3. – Godfrey 28/8, 2015 at 8:5

I have gnu-netcat 0.7.1-5 on Arch Linux. It's doc states that -l is needed for listen mode and -p to specify the port. The optional host and port arguments restrict connections to a specific remote host and port. There is no error because the command as written in the answer opens a connection that listens on a random port for connections from localhost:8000. – Fricke 28/8, 2015 at 14:52

@Fricke can you check that: while true; do printf '' | nc -lp 8000 localhost; done works for you? It seems to work on both version, if it does I'll update to use that instead, otherwise I'll just mention both commands. – Godfrey 28/8, 2015 at 15:41

Yes, while true; do printf '' | nc -lp 8000 localhost; done works fine. – Fricke 28/8, 2015 at 15:44

+1, very comprehensive. You may want to align the boundary string under "Therefore it is clear that:" with the one in the request – Mouldon 26/3, 2017 at 3:14

On OS X, nc won't accept both the -l and the -p arguments simultaneously. But this works for me: while true; do printf '' | nc -l 8000; done. – Lindalindahl 25/4, 2017 at 2:3

@Lindalindahl actually, -p is unecessary on Linux as well and I've removed it. Thanks for bringing that up. – Godfrey 25/4, 2017 at 5:37

For those who triy to construct that file upload message it's still not clear where/what new line chars need to be added – Firecracker 5/6, 2019 at 22:33

What is the purpose of putting so many dashes in the boundaries? Is it for easy visual recognition? – Sholokhov 9/10, 2019 at 16:10

@DanielPendergast not sure why this default. In general, to not conflict with the same number of dashes present in content of fields which would lead to ambiguity. – Godfrey 9/10, 2019 at 16:11

So far as I can tell, the point of putting ANY DASHES AT ALL in the boundary is to make it impossible to check the syntax of the request by eye. Please don't use them in your boundary tokens. – Whig 11/11, 2019 at 17:1

@DewiMorgan You are completely right. I edited the post and removed the dashes from the boundary string. – Ferd 7/5, 2020 at 10:11

@Ferd I rejected temporarily, what's the Firefox version you tested on? – Godfrey 7/5, 2020 at 10:12

@CiroSantilli冠状病毒审查六四事件法轮功 The point is not that dashes in the boundary string wouldn't work. They do work perfectly. But, as Dewi Morgan said: They are unnecessary and highly confusing because the multipart/form-data encoding requires setting "--" before each boundary and after the last boundary. – Ferd 7/5, 2020 at 10:18

@Ferd I understand. But I would like to stick to a concrete example from an specific browser in the answer. We should definitely mention the extra -- thing on the answer though. – Godfrey 7/5, 2020 at 10:22

@CiroSantilli郝海东冠状病六四事件法轮功 - why is it called "multipart" ? – Demean 1/9, 2020 at 22:37

@Demean this is explained on the book "The art of web development" by John Debber, please go and read it. Just kidding! I guess it's because each boundary= forms a "part". – Godfrey 2/9, 2020 at 6:35

@CiroSantilli郝海东冠状病六四事件法轮功 - haha. did you read my profile ? – Demean 2/9, 2020 at 6:37

@Demean "maybe" ;-) – Godfrey 2/9, 2020 at 6:37

Very informative post. As a little observation for those wanting to replicate everything, I believe the parameters of the netcat command are reversed, the host should come before the port (as in the referenced post), so in this case the command would be nc -l localhost 8000. Otherwise you get a nc: getaddrinfo: Servname not supported for ai_socktype error. – Maddie 4/12, 2021 at 20:55

111

enctype='multipart/form-data is an encoding type that allows files to be sent through a POST. Quite simply, without this encoding the files cannot be sent through POST.

If you want to allow a user to upload a file via a form, you must use this enctype.

Yuhas answered 24/12, 2010 at 12:49 Comment(5)

So.. if the file is not a binary file then can we work without this ? – Tardiff 27/8, 2013 at 0:1

From what I understand, you can use multipart/form-data for sending non-binary files but it is inefficient. I believe using application/x-www-form-urlencoded is the correct way to send non-binary data but someone with more experience with non-binary files may need to correct me. – Yuhas 27/8, 2013 at 9:36

The main advantage to using multipart/form-data for sending a file is that it will work automatically in both frontend and backend. You don't have to do any special handling. All files are binary even if they should only contain text. application/x-www-form-urlencoded is the standard way to POST a form without attached files. multipart/form-data is the standard way to POST a form with attached file(s). (There are also numerous other encodings, such as application/json and application/json-patch+json, which are common for communication between server and client.) – Cornerstone 19/9, 2013 at 17:34

Its worth pointing out you can base64 encode your image and send it as plain string data . – Ishmaelite 14/7, 2014 at 22:46

Further to @Prospero's comment above: you can absolutely send files via POST without using multipart/form-data. What you can't do is do that using an ordinary HTML form submission, without JavaScript. Setting a form to use multipart/form-data is the only mechanism that HTML provides to let you POST files without using JavaScript. I feel like this isn't clear enough in the answer, and that a naive reader might think that the inability to send files without multipart/form-data is a limitation of HTTP; that's not the case. – Corey 2/2, 2019 at 14:29

When submitting a form, you tell your browser to send, via the HTTP protocol, a message on the network, properly enveloped in a TCP/IP protocol message structure. An HTML page has a way to send data to the server: by using <form>s.

When a form is submitted, an HTTP Request is created and sent to the server, the message will contain the field names in the form and the values filled in by the user. This transmission can happen with POST or GET HTTP methods.

POST tells your browser to build an HTTP message and put all content in the body of the message (a very useful way of doing things, more safe and also flexible).
GET will submit the form data in the querystring. It has some constraints about data representation and length.

Stating how to send your form to the server

Attribute enctype has sense only when using POST method. When specified, it instructs the browser to send the form by encoding its content in a specific way. From MDN - Form enctype:

When the value of the method attribute is post, enctype is the MIME type of content that is used to submit the form to the server.

application/x-www-form-urlencoded: This is the default. When the form is sent, all names and values are collected and URL Encoding is performed on the final string.
multipart/form-data: Characters are NOT encoded. This is important when the form has a file upload control. You want to send the file binary and this ensures that bitstream is not altered.
text/plain: Spaces get converted, but no more encoding is performed.

Security

When submitting forms, some security concerns can arise as stated in RFC 7578 Section 7: Multipart form data - Security considerations:

All form-processing software should treat user supplied form-data
with sensitivity, as it often contains confidential or personally
identifying information. There is widespread use of form "auto-fill" features in web browsers; these might be used to trick users to
unknowingly send confidential information when completing otherwise
innocuous tasks. multipart/form-data does not supply any features
for checking integrity, ensuring confidentiality, avoiding user
confusion, or other security features; those concerns must be
addressed by the form-filling and form-data-interpreting applications.

Applications that receive forms and process them must be careful not to supply data back to the requesting form-processing site that was not intended to be sent.

It is important when interpreting the filename of the Content-
Disposition header field to not inadvertently overwrite files in the
recipient's file space.

This concerns you if you are a developer and your server will process forms submitted by users which might end up containing sensitive information.

Pecos answered 24/12, 2010 at 17:50 Comment(1)

The stuff about security after the most recent edit is all irrelevant to the question of what enctypes do. I know it's literally from the multipart/form-data RFC, but nonetheless it's an arbitrary dump of security considerations about submitting forms that are entirely orthogonal to whether data is sent as application/x-www-form-urlencoded or multipart/form-data. – Corey 3/2, 2019 at 9:51

enctype='multipart/form-data' means that no characters will be encoded. that is why this type is used while uploading files to server.
So multipart/form-data is used when a form requires binary data, like the contents of a file, to be uploaded

Sacrarium answered 4/7, 2013 at 9:13 Comment(0)

Set the method attribute to POST because file content can't be put inside a URL parameter using a form.

Set the value of enctype to multipart/form-data because the data will be split into multiple parts, one for each file plus one for the text of the form body that may be sent with them.

Cumshaw answered 25/9, 2013 at 11:53 Comment(1)

This implies that POST is likely to be sufficient for submitting a file via a form and that adding multipart/form-data is just a bonus in some vague way. That's not the case. Most files will absolutely require using multipart/form-data. – Harlen 11/6, 2017 at 12:33

enctype(ENCode TYPE) attribute specifies how the form-data should be encoded when submitting it to the server.
multipart/form-data is one of the value of enctype attribute, which is used in form element that have a file upload. multi-part means form data divides into multiple parts and send to server.

Oaken answered 27/12, 2015 at 1:29 Comment(2)

I believe enctype does not stand for encryption type. There is no encryption involved at this level. My guess is either encoding type or enclosed type. But surely it is not encryption type. – Interne 3/3, 2016 at 11:0

Your final bullet point here about <head> and <body> is irrelevant and confusing. – Corey 2/2, 2019 at 14:55

Usually this is when you have a POST form which needs to take a file upload as data... this will tell the server how it will encode the data transferred, in such case it won't get encoded because it will just transfer and upload the files to the server, Like for example when uploading an image or a pdf

Giffard answered 10/3, 2016 at 9:29 Comment(0)

-3

The enctype attribute specifies how the form-data should be encoded when submitting it to the server.

The enctype attribute can be used only if method="post".

No characters are encoded. This value is required when you are using forms that have a file upload control

From W3Schools

Offensive answered 11/11, 2018 at 22:52 Comment(1)

This quote doesn't even mention multipart/form-data. It's also pretty unclear; what does the sentence "No characters are encoded" even mean? -1. – Corey 2/2, 2019 at 14:22

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