Hello Ching,
If you want to validate the file extensions of the selected files for uploading before the uploaded process starts you can use client-side validation. Please, refer to
this help article which describes how to achieve client-side validation using the
validateExtensions() method and provides a sample code snippet. I suggest you also take a look at the
Client-side Validation online example.
If you have other questions, do contact us again.
Best regards,
Sophy
the Telerik team