I would say that this should be a won't fix.

It is valid mime type nosuch/mimetypeexists if you write the app that produce such files ?
https://stackoverflow.com/a/3828381/1000711

Nothing to fix here.

Wrong. nosuch/mimetypeexists is not a valid mime type until it is assigned and listed by the IANA.

[RFC2046] specifies that Media Types (formerly known as MIME types) and Media
Subtypes will be assigned and listed by the IANA.

Procedures for registering Media Types can be found in [RFC6838], [RFC4289],
and [RFC6657].

Additional procedures for registering media types for transfer via
Real-time Transport Protocol (RTP) can be found in [RFC4855].

Anyway, there is no good reason to spend a time on it. We does not developing an email server or anything else that require strict mime types validation.

anything else that require strict mime types validation.

LMAO that is literally the point of this field in the form. For security, to check strict for illegal mime types for security reasons hahaha

I agree though there there are more important things to spend time on ... ;-)

@brianteeman Please can you rewrite the sentence:

"A comma separated list of illegal MIME types to upload (blacklist)"

as it makes no sense at the moment.

LMAO that is literally the point of this field in the form. For security, to check strict for illegal mime types for security reasons

In our context "illegal mime" means "not allowed mimes", not illegal from IANA point of view ;)
This English so confusing ;)