what /should/ happen?  not sure.  there's only one place in emacs where
this comes up, lisp/net/eww.el in eww-submit.  in there, "filedata" is
associated with a string.

i did not find any code that calls to mm-url-encode-multipart-form-data
searching on the broader internet.

rfc2046 seems to say it's allowed to be any data that does not
contain the boundary.

from all of that, it seems reasonable to me to only accept string, and
raise deprecation for integer, and signal an error for everything else.

a comment says ";; How can this possibly be useful?" just above the
integer handling code.  so it seems plausible that handling an integer
here is not useful.

let me know if there's something else i can do to help!