John needs to attach a file to an automated vacation response.
Whatever this file may be (say, a PDF form), it would use the :mime parameter to the Sieve extension vacation as documented in Section 4.4 of RFC 5230.
This feature requires the UI elements in the short-circuited Settings -> Vacation as well as the Settings -> Filters dialogs.
Arguably depends on the resolution to T107 for the full monty.