Use signed email as method for deriving encryption key
The email signature would not do exactly what you want, but should prevent a man-in-the-middle attack on performing a Diffie Hellman key exchange through email content.

Client requesting decryption:

Generate random private key $CK_a$ 
Generate public key $CK_b$ 
Email $CK_b$ to server along with information on file needing decryption

Server:

Generate random private key $SK_a$ 
Generate private key $SK_b$ 
Generate session key $X$ from $SK_a$ and $CK_b$ 
Encrypt information needed to decrypt using $X$ 
Email $SK_b$ and encrypted payload to client

Client:
Generate session key $X$ from $SK_b$ and $CK_a$ 
Decrypt payload with session key, decrypt file

In this example the server is the one with the key to the file, swap server and client if you want the client to have the information for decryption, and the server to not. Other variants of this exchange exist, depending on where the file is located, and who should have access to the file, but since the file is at rest, it is already encrypted with a key that is not going to change.