Sorry for being a bit late with promised explanation.
First standards for POST method defined just one type of sending data: application/x-www-form-urlencoded
This way someone can send URLEncoded data. URL encoding means that chars like spaces and similar must be encoded before sent. Every such character should be replaced by % followed by the ASCII number of the character.
Anyway, with such method one can't send files. Later revisions of POST method introduces one more MIME type for POST - multipart/form-data.
This MIME type can be composed from other MIME types, where bound marks are used between the various MIME types sent.
Bound marks are random generated, and one bound should be used per POST.
Also the bound should be sent at declaring the MIME type of the POST, so that the server knows what bound mark is used.
This goes into HTTP headers sent:
MIME type: multipart/form-data, boundary=1234AB_my_unique_boundary
Data is sent like following:
--1234AB_my_unique_boundaryAs you can see, message is composed from two different MIME types, first one being a file to submit, and the 2nd one a value for a form's element.
content-disposition: form-data; name="file"; filename="some_file.zip"
**here goes the some_file.zip as binary**
Content-Disposition: form-data; name="some_form_element"
There is a boundary mark between the two.
Message can be composed from even more elements, each being of different MIME type.
So, it is pretty impossible to make a GUI that will generate such messages.
I can eventually make a text-box where someone will type such messages manually (inclusive entering the MIME types and boundary marks.