When enabling multi-file upload, you can control the concurrency by providing the `concurrencyLimit` property.

The `concurrencyLimit` property creates a `ConcurrencyRequester` instance, storing all requests in the instance's queue. The concurrent upload task quantity is restricted by the `concurrencyLimit` when using the `send` method of the instance at the end of the `uploadFiles` process.

- Use type imports in concurrencyRequest.ts for better clarity.
- Extract XHR load event handling into a separate function, onXHRLoad, in request.ts.
- Update references to onXHRLoad in both concurrencyRequest.ts and request.ts.