Torrent Freak notes that AT&T Mobility filed a patent (US 20140010082) on September 2013 for a system that would allow the wireless carrier to charge a subscriber more money for using file sharing, video, other other more intensive bandwidth services. Titled the "Prevention Of Bandwidth Abuse Of A Communications System", the patent operates on a credit system, whereby users erode their credit total by using select types of content and services.
"A user of a communications network is prevented from consuming an excessive amount of channel bandwidth by restricting use of the channel in accordance with the type of data being downloaded to the user," explains the patent. "The user is provided an initial number of credits. As the user consumes the credits, the data being downloaded is checked to determine if is permissible or non-permissible."
From the explanation, the system could be applied to any content or service AT&T deems "non permissible." "Various restriction policies also can be applied, such as levying additional fees and/or terminating the user's access to the channel" should they exceed their credit allotment or try to use content not authorized by AT&T under their current plan.
It's worth noting AT&T files a lot of patents for concepts they may or may not ever actually implement. On the other hand, this is only an extension of the kind of concepts AT&T has pushed for for years, including the company's recent controversial announcement of "sponsored data." AT&T isn't alone, billing and hardware companies have been pushing for several years for adoption of these kinds of systems, hoping to cash in on new contracts (see this 2010 presentation for example).
It's yet another AT&T idea aimed at further monetizing AT&T's network that won't be particularly popular among net neutrality advocates, as it puts AT&T in the role of both dictating "acceptable" Internet use, while imposing an entirely additional pricing layer. This being AT&T, it seems likely (if ever actually implemented) that such a system would be layered on top of the existing cap and overage bandwidth pricing model.
With net neutrality rules dead and regulators seemingly happy to approve such "creative" pricing initiatives, there's really nothing (other than consumer complaints and bad press, neither of which are guaranteed to keep a large company like AT&T or Verizon away from a bad idea) preventing their implementation.