Okay so there have been many occasions when you send a video or photo to a friend but then they tell you to delete the message or it will cause them to add up costs on there mobile plan/data.
I feel like that if there was an option in the setting's that you could set called "Download warning" (that allows you to set it to only give you a warning when you are on mobile data or both if you want.)
If this feature is going to be added maybe make a megabyte download limit slider where you can set example- (megabytes) 0, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10.
you slide the bar to the number and that's the limit when the message pops up.
it will bring up a message "this file requires to be downloaded" kind of like how the "spoiler" message pops up.
how to see the content you click on it like a "spoiler" post. (with a cancel button just in case of a miss click)