Issues/2168 comments support media

Closes #2168 
Adds support for images and media attachment in reader comment cells. 

Minor comment here (and again, no need to change anything!). You can use NSString's `.lastPathComponent` helper to get the extension, and afterwards, check using a NSSet. Plus, in swift, it gets extra cool because... you can even use `filter` and have this entire block in two lines.

Swifty!