Fixed crash on Asset View if file doesn’t exist on server

[snipe]

This is kind of a gross way to handle this, but it should generally be kind of an edge case. Basically, if the database thinks there is a file attached, but the file can't be found on the server, it would crash on the filesize() method. Probably better to handle this a little better at the controller method down the line, but this at least prevents the crash.