Hmmmmm, I'm not sure tbh of the implications of only sending binary. Certainly could be done.
I think (from a quick scan), the main purpose of MIME types if for receiving data. So if you're putting an image on the network, it's useful for the browser to know what that data is and what to do with it by default.
What do we lose if the metadata is set incorrectly
It's less that and more 'what do we gain when it is set correctly'?
But aye, I'm not really versed in this level of networking...