Allow providers to indicate supported mime types / media types

[roborourke]

In some contexts the media library can be opened with a media / mime type filter. Not every provider will be able to support every media type so we should filter the list of available providers depending on media type.

Proposed behaviour:

• Providers are assumed to support any media type by default
• Providers can override a method to return an array of supported mime types
• Provider should not be shown in list if it cannot provide the requested media types

Question: Should we support wild card or sub mime types like WP (sort of) does e.g. image/*? Would help.