How does HTTP file upload work?

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Let’s take a look at what happens when you select a file and submit your form (I’ve truncated the headers for brevity):

POST /upload?upload_progress_id=12344 HTTP/1.1
Host: localhost:3000
Content-Length: 1325
Origin: http://localhost:3000
... other headers ...
Content-Type: multipart/form-data; boundary=----WebKitFormBoundaryePkpFF7tjBAqx29L

------WebKitFormBoundaryePkpFF7tjBAqx29L
Content-Disposition: form-data; name="MAX_FILE_SIZE"

100000
------WebKitFormBoundaryePkpFF7tjBAqx29L
Content-Disposition: form-data; name="uploadedfile"; filename="hello.o"
Content-Type: application/x-object

... contents of file goes here ...
------WebKitFormBoundaryePkpFF7tjBAqx29L--

NOTE: each boundary string must be prefixed with an extra --, just like in the end of the last boundary string. The example above already includes this, but it can be easy to miss. See comment by @Andreas below.

Instead of URL encoding the form parameters, the form parameters (including the file data) are sent as sections in a multipart document in the body of the request.

In the example above, you can see the input MAX_FILE_SIZE with the value set in the form, as well as a section containing the file data. The file name is part of the Content-Disposition header.

The full details are here.

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