/usr/lib/mutt/mailto-mutt is in mutt 1.5.21-5ubuntu2.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 | #!/bin/sh
#
# mailto-mutt -- wrapper to use mutt as mailto handler from Iceweasel
#
# If you want Iceweasel 3.x to use this wrapper for mailto URLs, go to Edit >
# Preferences > Applications, and set "mailto" to "/usr/lib/mutt/mailto-mutt".
exec x-terminal-emulator -e mutt "$@"
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