How can I filter which files the user is able to select for the upload?

If you are using our Uppy file uploader you can set allowedFileTypes via the restrictions option

Even if you have already limited files on the client-side, you should also limit them on the back-end. For this, you can use Transloadit's file filtering capabilities.

See also:

Are there limits for upload time or processing time? Are there any discounts? What's the difference between Uppy and Transloadit?