In file and data_stream schemas, remove markdown specific formatting from descriptions of file.mime_type and data_stream. As the description may be rendered in different styles, it should not contain formatting specific to any markup language.

• Have you signed the contributor license agreement? y
• Have you followed the contributor guidelines? y
• For proposing substantial changes or additions to the schema, have you reviewed the RFC process? n/a
• If submitting code/script changes, have you verified all tests pass locally using make test? y
• If submitting schema/fields updates, have you generated new artifacts by running make and committed those changes? y
• Is your pull request against main? Unless there is a good reason otherwise, we prefer pull requests against main and will backport as needed. y
• Have you added an entry to the CHANGELOG.next.md?