Automatically set the extension of the files with truncated names

[AlexCSDev]

In the latest version files with truncated names are saved without a proper extension if the application's pretty dumb check (extension length > 4) fails.

Might be a good idea to try using something like Mime-Detective to give the file proper extension after it has been downloaded.

This issue might be moved into UniversalDownloaderPlatform in the future.