HTTP multipart/form-data multiple files in one <input>

Question

The background:

According to W3c, multiple files selected in a field, should be send by \"multipart/mixed\" type with separate boundary stri

Answer 1

I have confirmed what you found. I tested Firefox and Chromium, and this is what I get:

Content-Type: multipart/form-data; boundary=---------------------------148152952621447

-----------------------------148152952621447
Content-Disposition: form-data; name="files"; filename="fileOne.txt"
Content-Type: text/plain

this is fileOne.txt
-----------------------------148152952621447
Content-Disposition: form-data; name="files"; filename="fileTwo.txt"
Content-Type: text/plain

this is fileTwo.txt
-----------------------------148152952621447--

After an investigation, I found that the W3c information you provided is based on RFC2388, which is already made obsolete by RFC7578.

According to RFC7578 Section 4.3 (with my emphasis):

[RFC2388] suggested that multiple files for a single form field be transmitted using a nested "multipart/mixed" part. This usage is deprecated.

To match widely deployed implementations, multiple files MUST be sent by supplying each file in a separate part but all with the same "name" parameter.

So, your question:

How I should process the POST data?

My recommendation is ignore that W3c info and follow RFC7578.

Are there browsers that will send multiple files as a "multipart/mixed" or handling such case is not needed and I should simplify my code?

Very old browsers may use "multipart/mixed" but the usage is deprecated anyway, so no need to handle such case.

My recommendation: you should definitely simplify your code.