You sound as though you are opening the file you want as the attachment THEN going into email to set it as the attachment.
It does not work like that.
You dont send them the CONTENTS of the file, you send them the whole file.
1) Make sure the file you want to send is NOT open (so if you are sending a Word document make sure Word is NOT oipen).
2) Go into email, start a new email, then choose the attachment option (usually a paperclip).
3) Navigate to the folder where the file is and select it.
4) The name of the attachment will be added to the email (you dont see the contents, just the name of the file).
5) Write your email, address it, then press send.