Is it possible to change in file type filter restrictions?

I'd like to be able to post .cs files and have students submit them to the dropboxes. These .cs files are text files that contain CSharp source code. It is unclear why they are filtered... perhaps because they look like .css files. Workarounds include renaming files, zipping files, or pasting contents of sourcecode into other types of files, all of these add needless complication.