fakruzaruret one challenge is that apart from splitting different files between different templates and adapters, the MIME selection is also a security mechanism. Which MIME files you can safely accept on your server will vary from host to host. The way MIMEs are detected also varies from host to host. Flarum has no way to know about the particularities of those last 2 elements, this is up to the webmaster to make a sensible choice.
For absolute consistency and security, the extension should bundle its own file type checker (one is already used as a dependency for the fallback, but it doesn't support as many file types), and the extension should also proxy every single file instead of letting the webserver serve them with the execution rules that Flarum cannot know about or control.
If the files are hosted in the cloud we could potentially offer a less strict configuration, as it usually doesn't matter what the files are, for S3 it's just all binary. Then it's just a matter of properly isolating CDN domains and apply a CSP policy and you can even host HTML or javascript without any danger. This still wouldn't be very advisable though.
