Correct Dragging NSPasteboard File Promise

I've been sifting through the newer APIs and header comments for adding file promises to the pasteboard during dragging, and have a question. Assuming the code below, should I write something back to the pasteboard/item at the end of a file promise?

-(void)mouseDown:(NSEvent *)theEvent {
                NSPasteboardItem *p = [NSPasteboardItem new];
                [p setDataProvider:self forTypes:@[NSPasteboardTypeTabularText, NSPasteboardTypeString, NSPasteboardTypePDF, (__bridge NSString *)kPasteboardTypeFileURLPromise]];
                [p setString:kUTTypeXLS forType:(__bridge NSString *)kPasteboardTypeFilePromiseContent];
                NSDraggingItem *i = [[NSDraggingItem alloc] initWithPasteboardWriter:p];
                NSSize s = NSMakeSize(64, 64);
                [i setDraggingFrame:(NSRect){[self convertPoint:theEvent.locationInWindow fromView:nil], s} contents:[NSImage imageWithSize:s flipped:false drawingHandler:^BOOL(NSRect dstRect) {
                    [clipping drawInRect:dstRect];
                    return true;
                }]];
                [self beginDraggingSessionWithItems:@[i] event:theEvent source:self];
}

#pragma mark NSDraggingSource
-(NSDragOperation)draggingSession:(NSDraggingSession *)session sourceOperationMaskForDraggingContext:(NSDraggingContext)context {
    return NSDragOperationCopy;
}
#pragma mark NSPasteboardItemDataProvider
-(void)pasteboard:(NSPasteboard *)pasteboard item:(NSPasteboardItem *)item provideDataForType:(NSString *)type {
        NSMutableString *s = [NSMutableString string];
        if ([type isEqualToString:(__bridge NSString *)kPasteboardTypeFileURLPromise]) {
            static NSCharacterSet *set;
            if (!set) {
                NSMutableCharacterSet *c = [NSMutableCharacterSet characterSetWithRange:NSMakeRange('0', 10)];
                [c addCharactersInRange:NSMakeRange('A', 26)];
                [c addCharactersInRange:NSMakeRange('a', 26)];
                [c addCharactersInString:@"-_."];
                set = c.invertedSet;
            }
            NSMutableString *t = [self.window.representedURL ? self.window.representedURL.lastPathComponent.stringByDeletingPathExtension : self.window.title mutableCopy];
            [t replaceOccurrencesOfString:@" " withString:@"-" options:0 range:NSMakeRange(0, t.length)];
            NSRange r;
            while ((r = [t rangeOfCharacterFromSet:set]).location != NSNotFound)
                [t deleteCharactersInRange:r];
            PasteboardRef p;
            PasteboardCreate((__bridge CFStringRef)pasteboard.name, &p);
            CFURLRef u;
            PasteboardCopyPasteLocation(p, &u);
            CFRelease(p);
            [s writeToURL:[(__bridge_transfer NSURL *)u URLByAppendingPathComponent:[t stringByAppendingPathExtension:@"xls"]] atomically:true encoding:NSUTF8StringEncoding error:NULL];
        }
}