Send email with attachment

[Airy]

The documentation for that action says this:

https://wiki.keyboardmaestro.com/action/Send_Mail_Message

"To attach a file click the File Chooser button to the right of the action to select an existing file on disk. Option-click on the File Chooser button to show the currently selected file in the Finder (assuming it exists already). You can only attach a single file (if you need more than one file, it can be done with AppleScript)."

I think I found the answer here for you...

Attach Multiple Files in Email?

If the macro is being launched from the Finder, this can be done largely by taking advantage of built-in Copy and Paste capabilities. The macro shown copies the selected files in the Finder; has AppleScript activate Mail and make a new message; uses Keyboard Maestro to move to the body of the email and paste the files; Keyboard Maestro then moves the cursor to the beginning of the message body in case you need to add/change the message; finally, Keyboard Maestro deletes from the clipboard the …