The recent fix to accommodate posts of zero-byte files (and files of other
particular sizes) applied in v1.82 of curl seems to have a bug.
Specifically Curl_FormReader assumes that after readfromfile returns zero,
the next item in the form is non-file data, but it could be another file or
the end of the form. Here's a patch against v1.86 that seems to deal with
the issue better:

I haven't tested this particular patch yet (I've tested a similar patch
against an old version) -- I'm posting it now in case anyone notices right
away that it doesn't completely solve the problem or has something else
wrong with it.

Thanks in advance for any comments,

Dylan