[SMS App] Improve placeholder messages when the most-recent message has an attachment body
Summary
Improve placeholder messages when the most-recent message has an attachment body.
Also improves translate-ability.
Test Plan
Notice that a conversation which has a picture as the most-recent message used to say "image file" and now says Picture (the same as Android)
Notice that a conversation which has a video as the most-recent message used to say "video file" and now says Video
Other files are roughly the same. I swapped from taking the left part of the mimetype to taking the right since most files have the more-specific information on the right ("application/pdf", etc.) -- This is not test-able at the moment because the Android app does not send anything other than pictures and videos.