Adds a 'validate_media_type' option, default: true

Many people have complained that the Media Type Spoof Detection does not
work properly for their files. While whitelisting is available for some
files, it's not a general solution. This option leaves the "secure by
default" of the media checker in place, but allows people to turn it off
completely if they so choose.
Unfixed option name from previous commit
Be able to escale URL characters more easily

Including + now.