prefill the mimetype for ResourceProxy

[Dieter Van Uytvanck]

When adding a reference (ResourceProxy) of the type Resource, it would be nice if the mimetype of the URI could be automatically filled in by polling the web server.

[Willem Elbers]

Assumption: the mimetype for the resource is available (detected automatically or supplied by the user)

[Dieter Van Uytvanck]

Related: it would be good to add an attribute per Resource if the link points to a file or a landing page, so that a user (eg via the API) can filter out only those resources that can be machine-processed. The mimetype would not be sufficient to make a clear distinction between a landing page and a textual resource.