https://gitlab.freedesktop.org/xdg/xdg-utils/-/merge_requests/168
https://bugs.gentoo.org/974348
From: Brett A C Sheffield <bacs@librecast.net>

--- a/scripts/xdg-email.in
+++ b/scripts/xdg-email.in
@@ -71,7 +71,7 @@ run_thunderbird()
     fi
 
     if [ -n "$ATTACH" ] ; then
-        NEWMAILTO="${NEWMAILTO},attachment='${ATTACH}'"
+        NEWMAILTO="${NEWMAILTO},attachment='file://${ATTACH}'"
     fi
 
     NEWMAILTO=$(echo "$NEWMAILTO" | sed 's/^,//')