MIME stands for Multipurpose Internet Mail Extensions. An HTML version of the RFCs that define MIME are available at (http://www.oac.uci.edu/indiv/ehood/MIME/MIME.html).
In sum, MIME "redefines the format of message bodies to allow multi-part textual and non-textual message bodies to be represented and exchanged without loss of information." [RFC 1521]
v2.0 or later.
This question can be anwsered by reading about the MIMEFILTERS resource of the MHonArc documentation. The solution may require registering a pre-existing filter for the given content-type, or hooking in a new filter.
Yes. MHonArc's default filters are hooked in as described in the MIMEFILTERS resource.
Yes, but not recommended. Make sure you are familiar with how MHonArc does things before considering implemting your own multipart filter. In sum, you can completely replace MHonArc's filters with your own if you so desire.