MIME types have both standard types defined, plus a defined process for vendor extensions. Yes, via IANA.
The RFCs specifying what is needed before an IANA "designated expert" will accept a new Internet Media Type are a lot of documentation for a new programmer to read and understand, and my attempts to search the web for easier-to-digest introductory information from third parties weren't very fruitful. There's also a week's turnaround for this designated expert to make a decision. And if, say, the development of a new video game produces 20 different internal asset data formats used by the game and by its modding tools, would the designated expert appreciate having to review the registration of each of these formats as an Internet Media Type? I think I'm misunderstanding something very fundamental, and I know there's much I don't know.
Thirdly file types which have no additional requirements for registration, yet unambiguous are easy, by simply prefixing them with an already registered domain (usually reversed). e.g. com.google.whateverthefuckgooglewanttocalltheirnewfiletype.
Or io.github.some_username.some_projectname.some_type, right? I can get behind that in theory. But it'll take a lot of reengineering of container formats such as file systems and archives. Does FAT32, the default file system for removable storage media 32 GB or smaller such as USB flash drives and SDHC cards, support attributes such as content type? Wikipedia says FAT32 does not support extended attributes. Does exFAT, the default file system for larger removable storage media such as SDXC cards? Wikipedia does not say one way or the other. And Zip, a very common archive format, currently doesn't fully support extended attributes either and won't until Info-ZIP Zip 3.1 comes out.