If you're, like me, were trying to send a file and a JSON in the body togeter to some endpoint, then you should NOT put your JSON into request body, as when you don't send files. Instead, leave Body data field empty, use the parameters tab of Http request / Basic tab, and enter your JSON property name, as it is expected by server, put application/json
into Content-type and DON'T check URL encode?.
This is because when you send files, the HTTP request body is used for both your JSON and file data. They're separated by a "boundary string" and both of them must have a name for a server to understand which's where.
Sorry for a bit of necroposting, but this is the only meaningful result I met while googling for "jmeter send file with body".