.mht is an archive format created by Microsoft IE 8 when saving a webpage. It is essentially a mime multipart message.

My problem occurred when I uploaded such a file to a cgi-based server. The posted data would be fed to cgi.FieldStorage. (I can't post the file unfortunately)

As it turns out, cgi.FieldStorage tries to recursively parse the postdata, thereby splitting up the uploaded file; this fails. However, this (automatic) recursive behaviour seems unwanted for an uploaded file.

My proposal is thus to adapt cgi.py (line number for Python 3.2), so that in FieldStorage.__init__, line 542, read_multi would not be invoked in this case.

Currently it says:

    elif ctype[:10] == 'multipart/':
        self.read_multi(environ, keep_blank_values, strict_parsing)

Change this to:

    elif ctype[:10] == 'multipart/' and not self.filename: 
        self.read_multi(environ, keep_blank_values, strict_parsing)

(I apologise for not submitting a test case. When trying to create it, it is either very complicated, or not easily recognizable as valid. Moreover, my server used a 3rd party software (bottlypy.org: bottle.py))