"Open with" should be by file type first, mime type as fallback

Binary package hint: kwin

Often MIME types are incorrect on files, especially XML files. For example, an OPF file (which is part of many digital books) is an XML file, but is often encoded as text/plain. I want to open it with kxmleditor (always), so I right click, "open with", and choose ~"always do this" so that any time I click on an OPF again it opens in kxmleditor. This is fine, but now all of a sudden ALL text/plain files open in kxmleditor (even foo.txt, etc). This is clearly wrong, and counterintuitive. Even if the mime type *is* correct, there are different kinds of XML (for example) that I might want to open in different applications.

I think the strategy should be that "open with" looks to see if it has a mapping for the file type *first*, and then only secondly at the mime type. Additionally, when you click "always do this", there should be a dialog that says:

Which of the following should always be opened with _kxmleditor_:
[ ] all text files
[X] all .opf files

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