xdg-email --attach %p should be able to work with multiple attachments where %p is string of space-separated escaped paths.
Created attachment 61761 [details] [review]
Shell script for multiple attachments with thunderbird & claws-mail
Attached shell script does exactly that: attaching multiple attachments to an icedove, thunderbird or claws-mail compose window.
Please, feel free to put the code to good use in xdg-email as I have not figured out yet how to detect the default e-mail composer.