There is no such thing as a file extension name, it's a file name extension. It's called this way because file names could only be 8-byte strings initially and then this was extended with 3 extra byte places.

Neither part of the MIME type format even has to match any existing (commonly used) file name extension anyway. E.g. it can be `text/plain`. Even when it does it is just a coincidence (although very common), it actually references the format name (IIRC `image/jpeg` was used even when almost nobody were using `jpeg` for the extension and the convention was to use `jpg`).

> * file names could only be 8-byte strings initially and then this was extended*

At least part of that is not true. The ‘popular’ MS-DOS 8.3 form derives via CP/M from DEC OSes which used 6 character file names and 3 character file types, due to their use of RAD50¹ to fit 3 characters in a 16- or 18-bit word. The type field always existed, so the word ‘extension’ most likely refers to its presentation on the end of the file name, rather than an addition to a previous format.

¹ https://en.wikipedia.org/wiki/DEC_RADIX_50

This is interesting because compounds are mostly written together in other languages.

Never thought about it but file extension name really is a word. Someone replied saying this is three words, but it is not is it? It's an open compound word or maybe a "set phrase", I wanted to call it an idiomatic expression but that was clearly wrong.

The Wikipedia articles for "Set phrase" [0] and "Compound (linguistics)" [1] actually don't offer much of a distinction, so it's hard to say which of those is correct. Regardless, a compound with spaces ("open," as you said) is multiple words, not one word.

[0] https://en.m.wikipedia.org/wiki/Set_phrase

[1] https://en.m.wikipedia.org/wiki/Compound_(linguistics)

Who are you, Kath Day-Night?

I wanted to find an example from Kath & Kim, but couldn't find any on Youtube that wasn't just the "Look at moi" set-up to the actual joke. I'm sorry.