In short, the mail contains part
Content-type: multipart/appledouble;
and it contains next two parts
Content-type: application/applefile; name="Name of attachment.blah";
(resource) and
Content-type: application/octet-stream; name="Name of attachment.blah";
(data)
It corresponds to a Macintosh file format which has two parts: DATA (real content) and RESOURCE (arbitrary atributes). This is the reason why I see only one att. in Entourage (because Entourage understands AppleDouble) and two attachments in WebMail.
A nice description you can find on http://www.iana.org/assignments/media-types/multipart/appledouble (HERE).