Greg, the message you sent to bug-vm was corrupted, MIME-wise.
Here's what I saw
--[[image/jpeg
Content-Disposition: inline; filename="fish.jpg"][base64]]
/9j/4AAQSkZJRgABAQAAAQABAAD//gBIQ1JFQVRPUjogWFYgVmVyc2lvbiAzLjEwYSAgUmV2
OiAxMi8yOS85NCAgUXVhbGl0eSA9IDc1LCBTbW9vdGhpbmcgPSAwCv/bAEMACAYGBwYFCAcH
Some kind of tag is mixed in with the Content-Disposition
header. Also there should be a blank line between that
header and the start of the BASE64 goop. And there should
be a Content-Transfer-Encoding header. The Content-Type
header in the enclosing message was text/plain, which is
wrong if there's an attachment.
The usual suspects are TM and MULE, in that order. Both have
produced corruption os this type in the past, but I thought I had
nailed down all the MULE problems.