Added more detail to the Converter tests - harvested datastreams are now
checked for correct files, ids and mime types.  Only applied to the XML
converter and Labshare fixtures.
Added datastreams.csv files to the MIF text fixtures
Adapted both Converter test scripts to check datastreams against the fixture
data.
Both Converter modules now use the base Perl library MIME::Types to guess
the mime types of data files, rather than File::MimeInfo
Added a note about MIME types for datastreams, explaining how the existing
converter modules work and outlining what would need to be done if a data
capture scenario couldn't guess the MIME type from the filename extension.