When applying attachment in mail.app you are allowed to add your attachment anywhere in your body part. If you place it in between some text you end up with two attachment instead of one.  One with your acctual attachment and one with the text below where you placed your attachment in the mail.

Here is a ”solution” or should I say workaround at Apple support
http://support.apple.com/kb/TS2928?viewlocale=en_US

Symptoms

If you send messages with attachments via a Mail account that uses a Microsoft Exchange server, recipients of the message may see the text of the message as garbled, or split into several attachments.

Solution

As a workaround: In Mail, choose Attachments > Always Insert Attachments at End of Message from the Edit menu.

Additional Information

Tips

  • If you are sending only images as attachments, send the message as Rich Text–add some rich text formatting, such as bold or italicized text, or a bulleted list.
  • Alternatively, make sure to insert your attachments at the end of your message, after the body of the message.