Best practices (if any) to add metadata to encrypted files
This is pretty common. The method of encryption is public information, so it is safe to leave in the clear.

In your post you made no mention of authentication. This is something you absolutely must use, otherwise you have no way of knowing if your data has been modified and leaves you open to many attacks.

Consider using HMAC with SHA-256, or a block cipher mode with built-in authentication (e.g. GCM). Everything needs to be authenticated (any metadata, header, IV, ciphertext, etc).