allow configuration of upload adapter per mime type

[luceos]

I'd like to be able to specify the upload adapter that should handle a specific mime type upload. For instance:

• Images (mime: image/*) should be handled by imgur
• doc-x etc should be handled by google docs (to be implemented)
• other files should be stored locally

For this to work the admin page should allow for a flexible input list where we have two columns, the first column allows specification of the mime type and the second holds a dropdown of the available adapters; the same dropdown currently shown.

[luceos]

https://github.com/flagrow/upload/tree/mime-select

[luceos]

Fixed in 0.4.0