Fix the fix in r25439, which incorrectly assumed that normalized extensions are the...

Fix the fix in r25439, which incorrectly assumed that normalized extensions are the same length as their non-normalized forms. This isn't true: JPEG vs jpg for instance. :)
Additionally, makes the AJAX checks for non-exact matches work when an exact match isn't present; wfFindFile() checks local and shared repository, but returns no object if there isn't an exact match. Falling back to wfLocalFile() to get an object so the checks go through.