How to make sure encryption chiper type

Hi all, I used PGP Command line to encrypt some file. But i don't know exactly what the chiper type of my encrypted file.

It is true, if I set the chiper preferred first before i encrypt file. the result of encrypted file are using the last chiper which i set before i encrypt?

Because, For the case like i set the chiper 3des before i encrypt file, and after that i encrypt the file, so the result are using 3des algorithm right?

After that i want to test decrypt using aes-128, so i set the chiper aes-128 before decrypting process. But, why pgp still can decrypt the 3des encrypted file in other hand the key already set into aes128?

Anyone can give me some advice please?

Thank's.