Mime-attachment.html

Every time I send an attachment with my email message, the Mail application sends an empty Firefox file called "Mime-attachment.html" along with the attachment, one for each attachment (so that, if I send four attachments, then there will be four of these empty Firefox files). It's not a major problem, since the attachment is sent and the recipient receives it just fine. But I don't know why this "Mime-attachment.html" empty browser file is always attached, one for each attachment at that. Nothing like this ever happened when I was using Eudora as my email application. Does anybody know how to disable this on Mail and make it stop? Thanks!