The exception you are getting is because, the show file dialog must be initiated by an user's action - such as mouse button down or a Button's Click. If you are seeing this exception, this means that the link button's click is not considered as user initiated. Please try with a regular button.
Secondly, you would not be able to upload files and add more files to the same upload control. You can turn on the automatic upload feature
, which will start uploading files as soon as they are selected. You could also use a second instance of the upload control to browse and add the new files.
the Telerik team
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