Support 'file://' protocol

Change to support the file:// protocol (besides the http:// protocol) when downloading a file; the source file can be specified through e.g. 'http://host/dir/file.zip' or 'file://localhost/dir/file.zip' or 'file:///dir/file.zip'. Different protocol values (e.g. ftp:// ...) are not allowed. This is a requirement ot the NTE (text-proc) server.