This is a HTTP POST request sent with the request body specially formatted as a series of “parts”, separated by a boundary string. It allows a mix for form data, files etc to be sent as a HTTP POST.

Resources

https://ec.haxx.se/http-multipart.html

https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods/POST

Example 1

Form

<form action="my_post_page.php" method="post" enctype="multipart/form-data">

</form> 

html that is sent

POST /submit.cgi HTTP/1.1
Host: example.com
User-Agent: curl/7.46.0
Accept: */*
Content-Length: 313
Expect: 100-continue
Content-Type: multipart/form-data; boundary=------MyFormBoundaryvZ0ZHShNAcBABWFy
------MyFormBoundaryvZ0ZHShNAcBABWFy
Content-Disposition: form-data; name="fileToUpload"; filename="data.txt"
Content-Type: text/plain

Hello
------MyFormBoundaryvZ0ZHShNAcBABWFy
Content-Disposition: form-data; name="secret"; filename="file.txt"
Content-Type: text/plain

The contents of this file
------MyFormBoundaryvZ0ZHShNAcBABWFy--

The last boundary has “–” added to the end of it