How to process non-covertable pairs?

[ostasevych]

Hi! I found that it is not possible to convert from PDT to TXT, what leads to errors and even LibreOffice places locks.

Is that possible just to notify, that the conversion is not possible?

I also discovered that the conversion from PDF to ODT leads to the creation of wrong mimetype:
mimetypeapplication/vnd.oasis.opendocument.graphics instead of mimetypeapplication/vnd.oasis.opendocument.text. Thus the text becomes not editable in office editors.

Is that possible to fix?

Plus, it might be good to know the pairs which cannot be converted mutually.