Deep Cover

The passphrase opened the vault. Inside, four files: three numbered fragments (1, 2, and 3) and a note marked 0. The over-documenter numbered his secrets and labelled nothing else.

Fragment 1 has no extension and no note explaining itself. The note 0 had one line for the whole set:

"The carrier is never the cargo. Keep opening things."

Something is buried in the low bits of a file that is trying very hard to look boring. Whatever you pull out is another carrier: every shell holds a smaller one.

Recover what is hidden and submit it, lowercase.