Opening any file

NSOpenPanel *openPanel = [NSOpenPanel openPanel];
[openPanel beginWithCompletionHandler:^(NSInteger result) {
    NSURL *url = openPanel.URL;
    if (result == NSFileHandlingPanelCancelButton || !url) {
        return;
    }
    // do something with a URL
}];

Allowing opening multiple files

NSOpenPanel *openPanel = [NSOpenPanel openPanel];
openPanel.allowsMultipleSelection = YES;
[openPanel beginWithCompletionHandler:^(NSInteger result) {
    NSArray <NSURL *>*urls = openPanel.URLs;
    // do things
}];

Limiting to specific file types

NSOpenPanel *openPanel = [NSOpenPanel openPanel];
openPanel.allowedFileTypes = @[@".png", @".jpg"];
[openPanel beginWithCompletionHandler:^(NSInteger result) {
    NSURL *url = openPanel.URL;
    if (result == NSFileHandlingPanelCancelButton || !url) {
        return;
    }
    // do something with a picture
}];