Mail shows attachments in HEX

Question

I have two Mac's running Mountain Lion - my MBP shows mail attachments as expected (connected to the same gmail account) and my iMac insists on showing them as UUEncoded text. It's not all attachments - just some of them.
For exmple, things I send are fine, and emails recieved with graphical signautes show up fine. However some larger attachments like PDF's and DOCX documents often appear like shown below. These are identical systems both upgraded from Snow Leopard to Mountain Lion. There must be some difference, but I can't seem to find it.
Any ideas?
Tim
-------------000904090201090405000906
Content-Type: application/vnd.openxmlformats-officedocument.wordprocessingml.document;
name="TU100 =?ISO-8859-1?Q?=AD_SenseBoard_simulator_JW=2Edocx?="
Content-Transfer-Encoding: base64
Content-Disposition: attachment;
filename*0*=ISO-8859-1''%54%55%31%30%30%20%AD%20%53%65%6E%73%65%42%6F%61;
filename*1*=%72%64%20%73%69%6D%75%6C%61%74%6F%72%20%4A%57%2E%64%6F%63%78
UEsDBBQABgAIAAAAIQC4K8zjmQEAAEcGAAATAAgCW0NvbnRlbnRfVHlwZXNdLnhtbCCiBAIo
oAACAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
or another example
--20cf301cbdc8d8560104e71fd2d6
Content-Type: application/vnd.openxmlformats-officedocument.wordprocessingml.document;
          name="Quote.docx"
Content-Disposition: attachment; filename="Quote.docx"
Content-Transfer-Encoding: base64
X-Attachment-Id: f_hlz2im210
UEsDBBQABgAIAAAAIQAFSJxdrQEAAEoHAAATAAgCW0NvbnRlbnRfVHlwZXNdLnhtbCCiBAIooAAC
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

Answer 1

Quit Mail. Force quit if necessary.
Back up all data. That means you know you can restore the Mail database, no matter what happens.
Triple-click anywhere in the line below on this page to select it:
~/Library/Mail/V2/MailData/Envelope Index
Copy the selected text to the Clipboard (command-C). In the Finder, select
Go ▹ Go to Folder
from the menu bar. Paste into the box that opens (command-V), then press return.
A Finder window will open with a file selected. Move the selected file to the Desktop, leaving the window open. Other files in the folder may have names that begin with "Envelope Index". Move those files, if any, to the Trash.
Log out and log back in. Relaunch Mail. It should prompt you to re-import your messages. You may get a warning that the index is damaged and that Mail has to quit. Click OK.
Test. If Mail now works as expected, you can delete the file you moved to the Desktop. Otherwise, post your results.