additional email attachments

the following script has two problems;

1: Deletes an existing auto generated attachment

2: Will only add one email (multiple selections not allowed)

set the_files to (choose file with prompt "Select file to attach to Email. Sorry, ONE FILE ONLY." without invisibles)

tell application "Mail"

tell front outgoing message

make new attachment with properties {file name:the_files} at after the last paragraph

set visible to true

end tell

activate

end tell

Any thoughts on how to achieve a multiple selection dialog and for the insert not to delete my existing attachment.

thanks