[BibTeX] Handle various forms of file attachment data
Closes #734
Re https://forums.zotero.org/discussion/35909/bibtex-import-from-citavi-including-pdf-attachments/
and https://forums.zotero.org/discussion/28347/unable-to-get-pdfs-stored-on-computer-into-zotero-standalone/
Merge pull request #735 from aurimasv/bibtex-filepath

[BibTeX] Handle various forms of file attachment data