Improve file storage logic

[samwilson]

• Use BoxedCommand for shelling out.
• Remove separate preview logic (which wasn't working anyway).
• Correctly check for existing file, and return its URL.
• Use sha1 instead of MD5 for filename hash.
• Better error display (remove outer paragraph, so it gets .error class).
• Update required core version to 1.40.

[samwilson]

I'll chop this up into sensible separate PRs… this is just for discussion really… :-)