Bug 895726 - Ensure valid filename sent to Gallery view activity from SMS

Gallery will be in charge of maybe saving the file that we are passing.
If it receives a file that has no extension, trying to save inside the
'pictures' DeviceStorage will fail because of the missing extension. So
before initiating a new MozActivity from the SMS app, we check if the
filename has one and if it does not, we force one by guessing from mime
type.