Provide fallback 'mime' object that covers 80% case of data-uri usage.

This is the final result of the diff hinted at in #1123 (I moved it into a static object to enable future unit testing).

Throwing the error when `mime` is missing (like the `request` workaround) seems a bit blunt. This stays compatible with the browser version, while ensuring the most commonly-embedded types always work, regardless of the state of the dependencies.