8-bit characters mapped to 0xFF

It used to be that I could cut-and-paste 8-bit characters, i.e., anything
in the (decimal) 128-255 range (hex 0x80-0xFF) of my local character set
(which happens to be iso-8859-1), and exmh would do the Right Thing.  But
2.7.2 seems to mangle all such characters to 255 (0xFF).  Note that if I
attach a file and specify base64 or quoted-printable as the encoding, then
all is fine, but if I specify none as the encoding, or just insert a file,
or just cut-and-paste directly into a plain text message, then the mangling
occurs.

There seem to be plenty of changes in this general area, too many in fact
for me to isolate the problem so far.  I have studied the diffs between
2.7.0 and 2.7.2 in mime.tcl as well as seditExtra.tcl and thought I saw
some interesting candidates for the cause, but alas backing out the suspect
changes did not fix it.

Any clues?  This is 100% reproducible.

-- John