src/vs/workbench/browser/dnd.ts/DragAndDropContributionRegistry/register
class DragAndDropContributionRegistry:	register(contribution: IDragAndDropContribution): void {
		if (this._contributions.has(contribution.dataFormatKey)) {
			throw new Error(`A drag and drop contributiont with key '${contribution.dataFormatKey}' was already registered.`);
		}
		this._contributions.set(contribution.dataFormatKey, contribution);
	}