For every command there's a corresponding .desktop file in /usr/share/applications/ or $HOME/.local/share/applications/. The command is in the line starting with ´Exec=`.
For "Disk Image Mounter it's the file /usr/share/applications/gnome-disk-image-mounter.desktop. The ´Exec` line is
Exec=gnome-disk-image-mounter %U
so the command is
gnome-disk-image-mounter %U
where %U gets replace by a list of URLs for the files you have selected, for example
gnome-disk-image-mounter file:///tmp/myimage.iso file file:///tmp/other.iso
See the Freedesktop Desktop Entry Specification for more about .desktop files.
See the gnome-disk-image-mounter manpage for more about gnome-disk-image-mounter.