However, I thought a signature shall begin with '-----BEGIN PGP SIGNATURE-----'
The lines of Radix-64 (ASCII armored) that follows '-----BEGIN PGP MESSAGE-----' ... are they the message encrypted, or the message encrypted and signed, or just the signature? My clear text message is not included in the resulting file produced by the sign operation (this holds regardless whether or not 'Detached Signature' is selected).
The following outlines what I would expect:
A new file created with the following outline contents:
-----BEGIN PGP SIGNED MESSAGE-----
Here is my original clear text message preserved.
-----BEGIN PGP SIGNATURE-----
Lines of Radix-64 (ASCII armored) characters that make up the signature.
-----END PGP SIGNATURE-----