Just one comment. It seems like it would be better just to make add_method inline, since everything else in __init__ is, and it never gets called from anywhere else.
s/_method/_field/g
The thing I had in mind when I built the add_field method was that I could subclass FieldStorage and give the page handler more control over what is happening. This was convenient for the thing that got this all started: Posting large files. In fact, I intended add_field to replace the make_file function.
What I had in mind is that you override the add_field function to get context information (e.g. user name, upload directory and such) before the file upload comes in, so that you can put the upload where it belongs.
With the two callbacks, you can create the same effect, so it was no longer nessecary.
Also, the add_field is referred at least twice and I don't like duplicating code.
-- Mike Looijmans Philips Natlab / Topic Automation
