Support older XLIFF mime type (application/x-xliff+xml)

[goetzc]

The previous Marcel version that used Mimemagic had a different behavior then version 1.0 that uses Tika.

It seems that the mime type for XLIFF changed after spec version 2.0, from application/x-xliff+xml (private), to application/xliff+xml (standard tree).

From the spec:

XLIFF 2.0 was provisionally registered as application/xliff+xml in the IANA Provisional Standard Media Type Registry.

http://docs.oasis-open.org/xliff/xliff-core/v2.1/os/xliff-core-v2.1-os.html

Related: https://github.com/rails/marcel/issues/48

Thanks