To resolve the Eudora problem, the best solution is to fix Eudora.
Actually Eudora 3.0 is working fine as designed and MHonArc is treating
Eudora attachments pretty consistently with that proprietary design; only
the change in file name is a departure from what a Eudora mail sender should
expect a receiving mail client (Eudora or otherwise) to see. The design
model is that an attached file is transiently attached, to be detached at
the receiving end for use in a separate application program.
Eudora 4.0 introduced HTML message bodies. Probably an inline graphic is
tagged differently from an attached file containing a graphic.