将MIDI文件拖放到Finder工作正常但其他应用程序无效

Question

我正在尝试从我的应用程序中的NSOutlineView拖放到其他应用程序（.mid MIDI文件），它正在工作。我可以将它们拖到Reaper（DAW）中，然后将它拖到Finder中。但是当尝试像Presonus Studio One这样的其他应用程序时，它会在不显示加号的情况下拒绝它。如果我将同一个文件直接从Finder拖到Studio One，那么Studio One正在接受该文件，但不是来自我的应用程序。

我的代码应该可以工作，因为可以将文件拖到Finder和Reaper。

为什么这不起作用？我现在很困惑..

这是我的代码：

// The interface
@interface OutlineViewController : NSObject <NSOutlineViewDataSource, NSOutlineViewDelegate, NSDraggingDestination>...

@implementation OutlineViewController

- (id)init
{
    self = [super init];
    if (self)
    {
        // I have tried to call this from awakeFromNib but no difference
        [self insertAll];
    }
    return self;
}

- (void)awakeFromNib
{
    [super awakeFromNib];
}


- (void) insertAll
{
    [_outlineView registerForDraggedTypes:[NSArray arrayWithObjects:LOCAL_REORDER_PASTEBOARD_TYPE, NSStringPboardType, NSFilenamesPboardType, nil]];
    [_outlineView setDraggingSourceOperationMask:NSDragOperationEvery forLocal:NO];
    [_outlineView setAutoresizesOutlineColumn:NO];

    // HERE it will insert all lines to the outline view and it is working...

}
- (NSInteger)outlineView:(NSOutlineView *)outlineView numberOfChildrenOfItem:(id)item
{
    return !item?[_people count]:[[item child] count];
}

- (BOOL)outlineView:(NSOutlineView *)outlineView isItemExpandable:(id)item
{
    return !item?YES:[[item child] count] != 0;
}

- (id)outlineView:(NSOutlineView *)outlineView child:(NSInteger)index ofItem:(id)item
{
    return !item?[_people objectAtIndex:index]:[[item child] objectAtIndex:index];
}

- (id)outlineView:(NSOutlineView *)outlineView objectValueForTableColumn:(NSTableColumn *)tableColumn byItem:(id)item
{
    if ([[tableColumn identifier] isEqualToString:@"name"])
    {
        return [item name];
    }
    else if ([[tableColumn identifier] isEqualToString:@"parentName"])
    {
        return [item parentName];
    }

 return @"";
}

- (id<NSPasteboardWriting>)outlineView:(NSOutlineView *)outlineView pasteboardWriterForItem:(id)item
{

    // This is returning a valid NSUrl like
    //  file://localhost/Users/Me/File.mid on a drag
    // everything's working to Reaper and Finder
    return (id <NSPasteboardWriting>)[item fileURL];

}