Increase maximum file extension length from 3 to 15 characters
For Viewable, Testable, Compressible file types and download events, the file extension is no longer limited to 3 characters. 15 characters seemed like "enough". Similarly, a user's temp/QWK file type preference is also extended from 3 to 15 characters. This fixes issue #486, but file types such as *.tar.gz (2 dots in the extension) are still an issue and will be addressed next.