[Macworld.com]

Need to identify and filter Mail messages that bears attachments? Use your headers. [more]

[candace4]

Or you could use Mailsteward, a relational database that archives your email AND attachments.
www.mailsteard.com
Regards,
candace4

[iTuner]

Quote:

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Content-Type will contain something like multipart/related or multipart/alternative.

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There is also "multipart/mixed", which you'll see from time to time. Not to mention Content-Type "multipart/parallel", although I never see that one in practice.
So, if you want to filter for multipart messages, I suggest checking for "multipart/" and not worrying about the suffixes.

[spiderbat]

For those who are AppleScript-savvy, it is possible to trigger a script from a rule (let's say: if the sender is...) that looks for the presence of attachments and moves the corresponding messages to another folder.
(Disclaimer: I tested AS's ability to detect the presence of attachments, but I didn't implement a script with the full functionality, and since I know already about some nasty bugs in the AS implementation of Mail, I cannot warrant the working of every command.)