Thunderbird works different to outlook and other email clients and my friends who uses yahoo and hotmail email and i belive this affect outlook express to have problems looking at my attachments.

When I insert an attachment say a word file or a receipt.Thunderbird displays the image on the body of the message and not as a seperate file which is attached to an email like the other email clients.
When my friends receive this they cannot see the image

How do i change it so that the file which i want to send to my friends is just a seperate piece of document which can only be viewd by opening the file?