Define "mimetypes"

**Link to problem area**: https://spec.matrix.org

**Issue**
The spec references "mimetypes" all over (32 times, incl identifiers), however as far as I can tell without defining what a "mimetype" is.

I propose to
- define the meaning as "Media Types" (formerly known as MIME types) per [RFC2046](https://datatracker.ietf.org/doc/html/rfc2046),
- correct the spelling across the spec to either "media types" or "MIME types",
- reference the list of types https://www.iana.org/assignments/media-types/media-types.xhtml per [RFC2046](https://datatracker.ietf.org/doc/html/rfc2046) plus list any additional custom types the spec might introduce in a central place (e.g. appendix),
- leave existing identifiers untouched and continue to maintain it as a consistent name across the spec until an opportunity arises to refactor them comprehensively.

Start at the Matrix specification linked in the issue and search for all 32 occurrences of “mimetypes,” then read RFC2046 and the IANA media-types registry. Done means the term is defined, prose spelling is consistent, registry and custom types are referenced centrally, and existing identifiers remain unchanged.

Written by the indexing model from the issue text.