Tails

In Tails 0.19’s Iceweasel (at least), after clicking on a Tails detached signature link such as http://dl.amnesia.boum.org/tails/testing/tails-i386-0.19-rc1/tails-i386-0.19-rc1.iso.pgp, I am told it is a PGP/MIME-encrypted message header, and proposed to open it with Decrypt File.

The same happens with a update-description file hosted at tails.b.o: https://tails.boum.org/update/v1/Tails/0.16/i386/stable/updates.yml.pgp.

As a consequence, the signature files for the ISO image have been renamed with a .sig extension, and are now being served on tails.boum.org by default.

Does the IUK mechanism have to be modified to use .sig instead of .pgp extension as well?

No: these .sig files are not meant for human consumption.