custom cgi.FieldStorage make_file implementation doesn't get called

Question

I am building a file uploader that also shows the current progress, so I am trying to override the make_file method of the cgi.FieldStorage class, so that the current transferred data size can be logged. The problem is my method is not being called for some reason. Perhaps I misunderstood how this is supposed to work:

#!/usr/bin/env python
print "Content-Type: text/html\n"

import cgi
import cgitb; cgitb.enable()

class CustomFieldStorage(cgi.FieldStorage):
    def make_file(self, binary=None):
        print "make_file called"

form = CustomFieldStorage()
fileitem = form["file"]

Shouldn't the "make_file called" line be run?

Edit: apparently, it doesn't get called for very small files (< 1KB), but works for larger ones.