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

Question

The background:
According to W3c, multiple files selected in a <input> field, should be send by "multipart/mixed" type with separate boundary string and only one "name" parameter (as long, as the name should be unique in the form).
Writing POST data processing, I noticed that the major browsers send such multiple files as if they origins from different <input> elements, but with the same name. I.e. Instead of:
Content-Type: multipart/form-data; boundary=AaB03x
--AaB03x
Content-Disposition: form-data; name="files"
Content-Type: multipart/mixed; boundary=BbC04y
--BbC04y
Content-Disposition: file; filename="file1.txt"
Content-Type: text/plain
... contents of file1.txt ...
--BbC04y
Content-Disposition: file; filename="file2.gif"
Content-Type: image/gif
...contents of file2.gif...
--BbC04y--
--AaB03x--
...they send something like:
Content-Type: multipart/form-data; boundary=AaB03x
--AaB03x
Content-Disposition: form-data; name="files"; filename="file1.txt"
Content-Type: text/plain
... contents of file1.txt ...
--BbC04y
Content-Disposition: form-data; name="files"; filename="file2.gif"
Content-Type: image/gif
...contents of file2.gif...
--AaB03x--
The question:
How I should process the POST data? 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?
Notice: I am writing framework for handling HTTP, so using other libraries and frameworks is not an option.

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.