fix SuspiciousOperation in regex filters

If the path is prefixed with MEDIA_URL it will be replaced with MEDIA_ROOT (updated in 1b0ba0d)

Related to #251 

~~The SuspiciousOperation is raised here if the read of the image gets
permission denied. In many cases where this is wrong it is because the
path given is /media/... which may refer to MEDIA_ROOT not /media.~~