NSURL *url = [NSURL URLByResolvingBookmarkData:file.bookmarkData
                                       options:NSURLBookmarkResolutionWithSecurityScope
                                 relativeToURL:nil
                           bookmarkDataIsStale:nil
                                         error:nil];
[url startAccessingSecurityScopedResource];
...
[self.connection.remoteObjectProxy getDataFromURL:url
                                        withReply:aReply];

- (void)getDataFromURL:(NSURL *)anURL
             withReply:(void (^)(NSData *))aReply {
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
        aReply([NSData dataWithContentsOfURL:anURL]);
    });
}