Let me elaborate the FAQ entry mentioned in the top post a bit more. Monthly cellular, satellite and WISP data transfer caps of an end user is what “data limits” refers to.
Even if your chatmail relay provided an infinite amount of ephemeral space, every attachment you send to your peer would still have to be stored by them forever on their constrained device (unless you remember to set up all of your chats to use disappearing messages with a short timeout).
The default software of some cameras and phones produce an image that weighs up to 10MB even if the information content that the user would like to highlight could be perfectly represented in less than 100kB with proper cropping & scaling. The situation is even worse for videos.
You gave me an idea for how this could be solved in general: