Note: Later versions of Tails Live DVD ISO files (e.g. Tails v5.1, now have a UEFI-boot grub2 menu with a 'Tails (External Hard Disk) option. So just use a _.isodef.iso file extension and choose that option if you have an E2B USB HDD - otherwise choose one of the first options.
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This .grubfm menu can be added to the same folder as the ISO to get a 'User Menu' option when you select the .iso file. This allows us to UEFI-boot to Tails ISOs.
You must test on a real system (not a Virtual Machine) as it needs to detect a USB boot device.
Note: for Tails 4.6, and 4.16 you must delete the union=aufs parameter (why do the devs keep changing linux parameters every distro?).
As Tails seems to change on every version (!), you should look at the isolinux\live64.cfg file to get the correct 'params' parameters if you have a different version.
Tip: If you are booting from a VM or a Fixed-disk type of E2B USB drive (e.g. USB HDD or Corsair GTX flash drive), then you will need to press TAB at the Tails grub menu and edit the kernel parameters to remove the 'live-media=removable' entry each time you boot. For E2B, you can instead use the Tails_generic_for_USB_HDD.mnu from the sample mnu files folder (copy it and the ISO to the \_ISO\LINUX\MNU folder and edit the .mnu file with the name of your tails ISO file in two places).