Apple Support Discussions user willphillipson reports:
"Seemingly since my last update - to 10.6.2 - mail is doing something weird... Emails - particularly ones with attachments in them - are arriving in my inbox with the text part of the email message that follows an attachment (like an inline graphic) converted to an attachment - "ATT00001.htm". Additionally, if the email was a reply, then the included original text is another attachment - "ATT00002.htm".This issue can occur for any number of reasons, though an issue with Mail is unlikely. Sometimes Mac users will receive messages with these types of attachments from Windows users. In this case, if the messages are not truncated, the attachments can be probably be ignored. If you are receiving your messages via an Exchange Server, these types of attachments could be a result of SMTP used with it.
It appears the emails are truncated... I've been ignoring those pesky attachments!
Additional info: using hosted exchange as my email server. Experiencing the problem when receiving mail from others in my company."
ASD user daveywest suggests:
"Any message body after an attachment gets converted to the ATT0000X.htm, so the workaround is to ensure all your attachments are placed inline at the end of your message -- even after your signature lines.There are some settings in Mail that can help with this issue. In the Mail menu bar, select Edit > Attachments and be sure to check "Always Send Windows-Friendly Attachments" and "Always Insert Attachments at End of Message." If you continue to experience this issue, check with your Exchange Server administrator and be sure it is up-to-date. You might also try reinstalling the latest Mac OS X update using the Combo Update package.
If you want to test yourself, place an attachment in the middle of your signature and watch the first half of your signature show up, while the second half ends up in a "ATT0000X.htm" file.
This isn't a problem in the receiving mail reader, but a problem in the way mail is composing the message."