src/vs/base/common/dataTransfer.ts/VSDataTransfer/append
class VSDataTransfer:	public append(mimeType: string, value: IDataTransferItem): void {
		const existing = this._entries.get(mimeType);
		if (existing) {
			existing.push(value);
		} else {
			this._entries.set(this.toKey(mimeType), [value]);
		}
	}