MetropleX I get that so a certain point, but there is already an option in the app to auto-download media over mobile broadband. Using the app update solely as the reason for not allowing it doesn't hold it's weight.
Having the option, but set to default would be safe. If a user chooses to enable it various warnings of data use could be presented. Even just have it two-stage; one to alert of an update, and the second to download and install.
Signal is the only app that appears to deny updates this way.
Loving the username by the way - Transformers fan?