Greetings WrinklyNinja,
The mobile file can contain offline media, such as streaming content (movies, videos, music, etc) and other types of content for playback offline. If you have downloaded any movies or videos (I have done this before through my local Comcast service and Google) then you should use the appropriate app to remove the offline content that is no longer needed. Simply deleting the file may have adverse affects on your local media apps. A Factory Data Reset (as chrisinleedsuk mentioned) would certainly remove the file contents and return you to a factory fresh state, though that maybe overkill, since you will need to backup and restore all data and reinstall all of your apps.
-Doc
