This appears to be due to the Update KB945432. I first noticed it on JPEG
files from Mac users. After looking at the saved attachements with a hex
editor, it's clear that the "MacBinary" 128 byte header which is glommed onto
the front of the JPEG file when the attachment is sent by the Mac is not
being stripped off by outlook as it should be.
This is not limited to JPEGS, but is happening on any attachment sent from a
Mac. Any saved Mac attachment which I inspect in the hex editor as the 128
Byte MacBinary header attached.
Some applications are able to ignore the invalid header, but most cannot
handle it.
It's a major problem that MS should be putting at the top of their list.
Uninstall KB945432, or use a hex editor to strip the first 128 Bytes off of
any Mac attachment (works fine then). But it's pain.