Pegasus Mail & Mercury

I hope, I understood your problem.

I assume, that you want to create a new message with an attachement, right ?

If so, press 'F7' in the new-message editor, press the button for adding an attachment, you'll see an file-browser window opening,

where you can select the files on your disk.

Search for files using "My computer" and then drag and drop them on the message attachment window.