add filetypes controller to retrieve filetype

The controllers only action (create/POST) will retrieve
the filetype behind a given iri.
add fileextension mimetype mapping model.
add most important ext/mime mappings.
add ext/mime mappings to truncation exception.
provide fileextension in filetype response.
add javascript to provide filename suggestions

Provide filename suggestions if a filename has not been
set and the url-filename does not contain a fileextension.
Also provide suggestions if the filename that was set matches
the one provided by a previous suggestion.