[Mailman-Users] Change content filter rejection text

I'm wondering if there is any way to change the message sent by the content filter when a message is rejected. I'm trying to move away from letting users send HTML messages to the lists I manage, and instead of saying "The message's content type was not explicitly allowed" I would like it to explain that only plain text messages are allowed.

I saw there are language directories in messages/ with this field, but there are no English directories there (as I assume it's compiled into the program).

Is there any way to change this message?

Thanks.