Would it be possible to use gnome-fs-regular (system link to text-x-preview) or text-x-preview as a fallback icon as fallback icon before trying to load Thunar-fallback-icon.png? Thunar-fallback-icon.png is very out of place with most icon themes and text-x-preview is similar in design and provides the same meaning.
We no longer ship our own fallback icon, see commit 1546ba7e153afe23443b7ee0ecb0c483166e2642. *** This bug has been marked as a duplicate of bug 6662 ***