The MIME format is described in BodyFormats.{ps,txt,ez}.

The mailcap file format is described in Configuration.{ps,txt.ez}.

The metamail distribution is contained in mm.tar.Z.

A bunch of sample MIME-format messages are contained in the samples
directory, or in the compressed tar file of that directory called
samples.tar.Z.

Anything else you find in this directory is NOT SUPPORTED and 
generally best ignored.