Sure this is not still a pressing matter, but I figure I will share for anyone who stumbles across this in the future as I did.
For ItemAttachments you need to load the MimeContent for the item, then you can simply write to the file/output [".eml", ".msg"]:
if (attachment is FileAttachment)
{
FileAttachment fileAttachment = attachment as FileAttachment;
// Load attachment contents into a file.
fileAttachment.Load(<file path>);
}
else // Attachment is an ItemAttachment (Email)
{
ItemAttachment itemAttachment = attachment as ItemAttachment;
// Load Item with additionalProperties of MimeContent
itemAttachment.Load(EmailMessageSchema.MimeContent);
// MimeContent.Content will give you the byte[] for the ItemAttachment
// Now all you have to do is write the byte[] to a file
File.WriteAllBytes(<file path>, itemAttachment.Item.MimeContent.Content);
}