MIME encoding is not a form of stuffing. All email programs, with no exceptions, always encode all email attachments, using typically one of three encoding methods: MIME (used by 95% of all email programs, including AOL, Outlook, Entourage, Mail.app, and so on), UUencode, or BinHex.
AOL *also* compresses attachments if you attach more than one file, or if you attach a folder. It's based on the number of files, not on the size of the attachment. One file: MIME encoding. Multiple files, or one folder: MIME encoding plus compression.
Now, here is where it gets complicated:
Mac files have two "parts," called a "resource fork" and a "data fork." Other computers do not use or understand two-part files.
Apple has invented a type of encoding called AppleDouble to handle two-part files. The file is broken into TWO files: One part of the data fork, one part for the resource fork plus special Mac information (icon, Type code, Creator code, and so forth).
Each of those two parts is encoded using MIME. Then both parts are emailed.
Some mail programs do not understand AppleDouble. The person who receives the email sees TWO attachments, not one. One attachment is the file. The other attachment is the resource fork and Mac information. The first attachment is useful, the second attachment can not be used on a PC and sould be ignored.
AOL gets very confused by AppleDouble emails. AOL will not show you the attachment. Instead, if you send an AppleDouble attachment to an AOL user, the AOL user will see this:
"This email has an attachment encoded using MIME. AOL can not decode the attachment. To decode the attachment, you will need to download the email and then use a decoder program. For more information, visit Keyword: MIME."
The message means exactly what it says. The attachment can't be decoded by AOL. In order to use it, the AOL user will need to download the email and then run a MIME decoder program. (I believe the newer versions of Stuffit Expander can decode MIME.)
Unfortunately, there is no way to turn off AppleDouble in Mail.app. As a result, I don't use Mail.app.