Prevent video plugin from processing svg [#934]

Prevent video plugin from processing svg [#934]

Prior to the gstreamer-1.0 upgrade, the video processing engine included
a check for excluded extensions which gstreamer might accept despite us
not wanting to process them. In commit 91f5f5e, the check against
EXCLUDED_EXT was removed. Since then, the video plugin has accepted and
attempted to process svg files.

This commit adds the check against EXCLUDED_EXTS into the sniff_handler
function so that we can bail out on certain file extensions before the
plugins tries to sniff the file type. The previous implementation
excluded nef files, which appears to be a Nikon camera image. I've
copied that forward to this code. I've also added a log message to
indicate that we're purposefully refusing to process the file.