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If you're using Outlook Express, maybe the following
info will help:
Go to...Tools / Options / Security tab...and uncheck...
"Do not allow attachments...etc..." / Apply / OK.
If that's not the fix...read these KB articles:
(326675) "No Preview Available" and "File
Format Is Not Supported" Error Messages
Occur When You Try to View Graphic Files http://support.microsoft.com/?kbid=326675