The Qt platform does not currently have MHTML enabled, and even if enabled, it will not run the tests or open .mht documents. The only reason it doesn't work is because we do not recognize the .mht extension.
Created attachment 173317 [details] Patch
I have one uncertainty about the patch. We are not skipping the mhtml tests in TestExpectations, so I am not sure were they are automatically skipped or unskipped, but they run and pass.
Comment on attachment 173317 [details] Patch Attachment 173317 [details] did not pass qt-ews (qt): Output: http://queues.webkit.org/results/14779451
Comment on attachment 173317 [details] Patch That error does not look related to this patch in any way.
Comment on attachment 173317 [details] Patch Clearing flags on attachment: 173317 Committed r134214: <http://trac.webkit.org/changeset/134214>
All reviewed patches have been landed. Closing bug.