Allow remote method to accept uploaded file(s) via regular "accepts" parameters.

So far, methods accepting files have to parse them manually from the request object, see loopback-component-storage for an example implementation.

Possible flavours to support:

• Access the full request as a single "file" (see e.g. Add support for coercion by endpoints with type File strong-remoting#318):
  {arg: 'file', type: 'file', http: { source: 'body' }, required: true}

• Accept multiple files uploaded in a multipart body (see e.g. Attachments in apis #3264 and Formdata not available when upload file #3171), this should simplify loopback-component-storage implementation