FEATURE: Onebox can match engines based on the content_type (PR #13876)

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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