Would this really be lovely? Isn't there some value in having 3 distinct formats (still picture, moving picture, moving picture with audio)? Each one is a subset of the next, but they are 3 different things that can easily be identified by what they (only potentially, but probably) are.
It's a win from the implementation side because you can support all three use cases with one format, reducing code duplication and at least hypothetically leveraging the same improvements across the three use cases with no additional effort.
The UX concern you're describing doesn't necessarily have to have anything to do with the implementation details themselves, as demonstrated by sites like imgur rebranding silent mp4/webms as "gifv". As long as the new format makes it reasonably easy to access data about its content (e.g., header field indicating animation/movement), there shouldn't be any issue with collapsing the use cases into a single implementation and simultaneously addressing the UX concern you mention.
