Another tweak for 'SL-15547: Viewer hung while looking for a file in cache' -...

Another tweak for 'SL-15547: Viewer hung while looking for a file in cache' - this time based on Henri's suggestion in this discussion: https://bitbucket.org/lindenlab/viewer/commits/e28c1b46e9944f0215a13cab8ee7dded88d7fc90#comment-10537114