Thanks for clarifying!
> This part is exactly what I'm asking about: how do I "upload the file"
> using EWW? Can you please show what I should type to do that?
0. In a terminal, do the following
#+begin_src bash
git clone
https://git.sr.ht/~ozzloy/emacs-bug-63941 cd emacs-bug-63941
git checkout reproduce-bug-63941
./server.py
#+end_src
1. In an emacs window, do the following
M-x eww<Enter>
localhost:8085<Enter>
Once EWW opens localhost:8085, it will display the web page with a button
labeled "Browse". Click that button.
2. It will then ask you to choose a file. Choose the file
=.../emacs-bug-63941/filename= in the same directory where =server.py= is.
3. Hit <tab> to go to the "Submit" button, and hit <Enter> to upload that file.
> Where does one see this result? Is it in some Emacs buffer? In that
> case, what is the name of that buffer?
The output will be in the terminal where =./server.py= was run. I ran it in an
emacs shell buffer named =$emacs-bug-63941=.