Categorize browser built in mime-types:

[jordwalke]

Firefox: https://dxr.mozilla.org/mozilla-central/source/uriloader/exthandler/nsExternalHelperAppService.cpp#403

when browser sees an iframe with an extension, it uses that and it seems impossible to get it to treat it any differently.

[jordwalke]

https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/MIME_types
Mime sniffing: https://mimesniff.spec.whatwg.org/

[jordwalke]

"If the resource is retrieved directly from the file system, set supplied-type to the MIME type provided by the file system.
If the resource is retrieved via another protocol (such as FTP), set supplied-type to the MIME type as determined by that protocol, if any.
[FTP]

If supplied-type is not a MIME type, the supplied MIME type is undefined.
Abort these steps.

The supplied MIME type is supplied-type.
"

[jordwalke]

https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/MIME_types/Common_types