* FEATURE: Onebox can match engines based on the content_type
`FinalDestination` now returns the `content_type` of a resolved URL.
`Oneboxer` passes this value to `Onebox` itself. Onebox engines can now specify a `matches_content_type` regex of content_types that the engine can handle, regardless of the URL.
`ImageOnebox` will match URLs with a content type of `image/png`, `jpg`, `gif`, `bmp`, `tif`, etc.
This will allow images that exist at a URL without a file type extension to be correctly rendered, assuming a valid `content_type` is returned.
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