UIDocumentInteractionController显示文件名和文件类型，无法查看文件内容。

Question

浏览Stackoverflow但无法知道为什么文件无法打开。我已经在本地设备磁盘上下载了该文件。 UIDocumentInteractionController打开预览视图，但只显示文件名和文件类型。以下条件

1.使用Xcode 6.1.1

2.在Device 5s OS 8.1.2上进行测试

3.以下是我写的代码。

- (UIViewController *) documentInteractionControllerViewControllerForPreview: (UIDocumentInteractionController *) controller {
return self;
}
- (UIView *)documentInteractionControllerViewForPreview:
  (UIDocumentInteractionController*)controller
{
 return self.view;
}

- (IBAction)downloadFile:(id)sender {

HUD = [[MBProgressHUD alloc] initWithView:self.navigationController.view];
[self.navigationController.view addSubview:HUD];

HUD.delegate = self;
HUD.labelText = @"Loading";

[HUD show:YES];


NSURLRequest *request = [NSURLRequest requestWithURL:[NSURL URLWithString:fileURL]];
AFHTTPRequestOperation *operation = [[AFHTTPRequestOperation alloc] initWithRequest:request] ;

NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *path = [[paths objectAtIndex:0] stringByAppendingPathComponent:fileName.text];
operation.outputStream = [NSOutputStream outputStreamToFileAtPath:path append:NO];

[operation setCompletionBlockWithSuccess:^(AFHTTPRequestOperation *operation, id responseObject) {
    NSLog(@"Successfully downloaded file to %@", path);

    [HUD hide:YES];
    NSURL *URL = [NSURL fileURLWithPath:[NSTemporaryDirectory() stringByAppendingPathComponent:fileName.text]];

    NSLog(@"URL %@",URL);
    self.documentInteractionController = [UIDocumentInteractionController interactionControllerWithURL:URL];

    // Configure Document Interaction Controller
    [self.documentInteractionController setDelegate:self];

    // Preview PDF
    [self.documentInteractionController presentOpenInMenuFromRect:CGRectZero inView:self.view animated:YES];
} failure:^(AFHTTPRequestOperation *operation, NSError *error) {
    NSLog(@"Error: %@", error);
}];

[operation start];
  }