Bugfix for buggy mime type resolution in Win32 environments

Current builds of Evince for Windows (e.g. the one provided by msys2) are unable to open PDF files. Unfortunately, that renders the application completely useless. The primary reason for this is that Evince's internal mime type "application/x-ext-pdf" cannot be resolved in Windows' regedit. Hence, no backend can be found for the document. This bugfix tries to be as minimally invasive as possible by removing "x-ext" from the resolved mime type before proceeding with the lookup in Windows' regedit.