In a current project we ran into a curious problem, every HTML Mail has an file attachment named "attachment.dat".
After some research in the "mimemail.theme.inc" (function "template_preprocess_mimemail_message()"), we figured out that the problem is located inside the $css content.
In our case the implementation of the google webfont produces this issue (@import url(http://fonts.googleapis.com/css?family=Open+Sans:400italic,600italic,400...)). Basicly @import has to work inside html mails, so mimemail seems to do something wrong here?
Whatever. We can work around this problem, using the <link ...> - implemantation inside the mimemail-message.tpl file instead of the declaration directly inside the css code.