pgp message extensions

[vedaal]

have noticed that GnuPG distinguishes between types of pgp messages in
accepting the .txt extension:

clearsigned or armored signed messages saved as .txt, will be accepted for
verification and reading.

encrypted pgp messages saved as .txt files will not be opened by GnuPG
{windows command line version, 1.06}

{gpg output copied from dos window:}

Welcome, Honored Traditional User,
 What is thy bidding?
 Type a valid Command, and it shall be done.
 C:\gnupg>gpg --decrypt d:\tcve1.txt
gpg: can't open `d:\tcve1.txt'
gpg: decrypt_message failed: file open error

simply changing the extension to .asc and trying again, works perfectly

are there any other extensions that can or should be tried  in the event of
a similar error message?

tia,

vedaal