Attachment goint as dat file

Using Outlook 2000 - when I attach an excel file to outgoing mail, it arrives
as a .dat file and the recipient can't open it. If they reply back to me and
I reply to them, they then receive it as the excel file and can open it.

When I attach the file, it shows up in the body of the email as an excel
icon. When it goes back on the final 'reply' it's listed at the bottom of
the email. (this happens with any attachment)

Pls help.