Migrating Mail from Lion to Mountain Lion leaves behind a folder containing previous attachments.
Attachments in Lion were saved to ~/Library/Mail Downloads, but under Mountain Lion, Mail is now sandboxed, and the new path is ~/Library/Containers/com.apple.mail/Data/Library/Mail Downloads.
During the upgrade, the folder contents are copied from the old location to the new, so the old location can be deleted to save some space.
[kirkmc adds: While this may not save a lot of space, if you get a lot of e-mail with attachments, you may have plenty of files in that folder. I clean mine out from time to time, so it's good to note the new location.]
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