Infer extension from `content-type`

[benmccann]

Clear and concise description of the problem

A static file fetched from a URL like https://cdn.example.com/f2d0003e won't be saved with the proper extension

Suggested solution

Map content-type to extension

Alternative

No response

Additional context

This is very low priority for me. It was just something I happened to notice

Validations

• [X] Follow our Code of Conduct
• [X] Read the Contributing Guide.
• [X] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

[antfu]

Sounds solid. Do you mind sending a PR? Thanks!