Addresses #460, importFromURL fails when importing PDFs from servers that do not properly support HEAD requests
Now inspects supposed PDFs after download and deletes if not actually PDF format
Also:
- Fixed bug when running importFromDocument() on a PDF on Windows that would result in an incomplete or missing (since r1688) attachment item
- importFromDocument() no longer returns an itemID, since it can be partly asynchronous now
- Added rudimentary 'text/html' support for Zotero.MIME.sniffForMIMEType()