Hello Pete,
Unfortunately we do not have any control over the "browse" dialog window and how the browser chooses to interpret the filetype filter property. The thing we can control is how RadAsyncUpload processes the chosen file. In spite of the fact that you can see other file types in the window, if you try to upload one of them, RadAsyncUpload will mark it as invalid. Only the allowed extensions will be uploaded.
Kind regards,
Bozhidar
the Telerik team
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