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 [[http://tools.ietf.org/html/rfc5230#section-4.4 | Section 4.4 of RFC 5230]].
Arguably depends on the resolution to T107 for the full monty.