Hello John Chatt,
RadUpload can't filter the files so that only the allowed ones are shown. It is that such features are not provided by the browsers. As you have noticed, the files are validated after they were uploaded. If you want to check the extensions on the client, please follow the advice shown
here. It is for RadUpload for ASP.NET Ajax, but with slight code changes it will work for RadUpload Classic as well.
What you need to change is:
1) use GetRadUpload instead of $find
2) Use ValidateExtensions instead of validateExtensions.
All the best,
Genady Sergeev
the Telerik team
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