Nautilus freeze break request

[Alexander Larsson <alexl redhat com>]

There is a memory use problem in nautilus:
https://bugs.launchpad.net/ubuntu/+source/nautilus/+bug/204413

On larger zoom levels we read the actual image files to use as
thumbnails. However, we forgot to check that the file is an image
before we do this, so we can accidentally start loading other types of
files too. This is especially problematic since the thumbnail size limit
only affects images, so these may be large files.

Its not exactly a leak, as the memory will eventually be freed, but for
very large files it can be a problem anyway.

Fix attached.