Sub extension fix

Hi,
I noticed that if you call addic7ed somefile.avi, but the avi file is not in the directory,
then the subtitle is dowloade with the name of the file, rather than with the correct .srt extension.

E.g.:

addic7ed an_episode_name.avi

downloads the subtitle as "an_episode_name.avi" rather than as "an_episode_name.srt".

This commit fix this.