We did receive a report, that on a Windows Phone 8.1 there is a message that the html5 overideMimeType is not supported. This message does mean that the dictionary files, which are locally stored on the device in the html5 application cache, cannot be read, i.e. translations will not work.
The DfM WebApp runs nicely since version 11 of the Internet Explorer; Internet Explorer versions prior to 11 (e.g. Internet Explorer 9 and Internet Explorer 10) had a poor support for HTML5 and the DfM WebApp does not run on those older Internet Explorer versions.
What version of Internet Explorer is running on Windows Phone 8.1 ?