Add *.* to the Import File Filter Dialog

[Siedlerchr]

From the forum:

http://discourse.jabref.org/t/importing-bibtex-files-with-jabref-4-1-on-macos-sierra-10-12-6/1060

Currently JabRef only allows the import of files with the list of supported file extensions.
So, as there is now file extension .bibtex for bib files import we should add a . (All File types) filter.
The import should then use the automatism (as it's now the case for all avaiable import formats)