Make the transformation of TEI files into HTML dependent on the MIME type

Make the transformation of TEI files into HTML dependent on the MIME type. Use cases:

1. The normal TEI files could use the default transformation, but ATF files could use a different XSLT stylesheet defined in portalconfig. This would be the case for CoNSSA.
2. The researcher could define two different XSLT stylesheets in the portalconfig, one for their normal TEI files, the other for the ATF files.