Google says "you can send and receive messages up to 10 megabytes (MB) in size" in Gmail. "However, the precise amount allowable will depend on the attachment. When you add an attachment, the size of a file may increase because transport encodings are automatically added. (Transport encodings are the information that allows your message to be safely sent and read.)"
Sometimes Gmail lets you attach files whose total size exceeds the 10 MB. So I managed to attach more than 12 MB.
Did you manage to attach more than 12 MB in Gmail?
Update: From May 2007, the maximum attachment size in Gmail is 20 MB.
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