Update error messaging to indicate file extension rather than MIME type

What does this MR do and why?

#335278 (closed) received some attention on HackerNews and many users pointed out that the error message does not use correct terminology.

What we are forbidding are file extensions (e.g. .json, .html, .mov, etc.), and these are associated with a MIME type (e.g. application/json, text/html) but are not MIME types themselves.

This MR updates the error message to use file extension rather than MIME type.

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.