Update username error message to emphasize on 'reserved file extension'

What does this MR do and why?

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This is a follow-up from !70374 (diffs) which changes the error message for usernames to file extension types (previous: MIME types). This MR adjusts the messaging to a reserved file extension from feedback on HackerNews, emphasizing on reserved.

cc @bwill @dfrazao-gitlab @mkaeppler

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