I just happened to click upload button on slideshare.net (http://www.slideshare.net/upload) to upload a presentation. And, I was presented with this - http://4.bp.blogspot.com/_Jg5VhRmvVtY/SbKCFlSRlsI/AAAAAAAAAiE/mjt3Hcu4754/s1600-h/problem.png

Other information:
Now, that's a huge dialog covering my screen. Just imagine if a web app needed to filter out any of 60 file-extensions! This dialog box would widen so much that it might eat up my whole screen. Now, won't that be annoying when you've to upload many things one by one.
A good way would be to trim out the list of file-types and end it with a '...' to limit the widening. When the user clicks over the file-type filter, the pop-down can show all the types.
I'm using Ubuntu 8.10 Intrepid Ibed (64-bit) with Firefox 3.0.3 and Gnome 2.24.1
I wonder if this should be done in gtk itself or should firefox be hacked for it.