Recognize Windows network shares on file attachment import

E.g. in bibtex `file = {Test:\\\\HOST\\share\\test.pdf:application/pdf}`

Related to zotero/translators#735 and we've seen a bibtex file like this [posted on the forums](https://forums.zotero.org/discussion/25145/problem-importing-pdfs/) before.

Also, anchor drive letter to beginning of string (though I don't think a colon could be anywhere else anyway)