In file selection dialog, make the window resizable or make "file types" combobox shorter in width. It's not necessary to list ALL the supported file types. If there are MANY file types/extensions, the dialog window will become too long/wide and can't fit in screen.

To do this, we can borrow the design from MS Windows. In Windows, I believe the file selection dialog is popped up with its preferred size initially and if there were too many file types that can't be fully shown in file types combobox/drop-down list, the dialog lists only few types like this (*.jpg; *.bmp; *.png;...) and if you move mouse pointer over it, the full list will be shown as tip text. This way keeps the dialog relatively small, so that it fits the screen well.

Other information:
I found this usability issue when I was trying to upload photos to flickr.com. To reproduce it, try the following:

1. Go to http://flickr.com and create a free flickr.com account if you don't have one.
2. Login to your account and click the link "Upload Photos and Videos".
3. and then click "Choose photos and videos". You will see a very long file selection dialog (~= 1556 pixels in width).