It looks like that file is MIME-encoded “multi-part” file that contains (at least) two components files.
You can decode the multi-part using a MIME decoder.
The first component file looks like a simple “name=value” property file
The second component file is XML.
I don’t know if this is relevant, but a search for Mascot file format
gave me this reference page:
Also, there were some interesting hits when I searched for parser "x-mascot"
and parser mascot
. If you can find an existing parser, then you may be able to save yourself implementation effort.