At present, when an invalid file is selected, the alert message does not clearly indicate what is the validation error - invalid extension or the size being too large. It should show a message that clearly states what the problem is so the user can fix it.


For the time being you can handle the AsyncUpload's OnClientValidationFailed event and show different messages—http://docs.telerik.com/devtools/aspnet-ajax/controls/asyncupload/client-side-programming/onclientvalidationfailed.

Example:

ASP.NET
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<telerik:RadFileExplorer runat="server" ID="RadFileExplorer1">
    <Configuration ViewPaths="~/" UploadPaths="~/" DeletePaths="~/" SearchPatterns="*.jpg" />
</telerik:RadFileExplorer>

<script>
    // Prevent the built-in validation alert
    Telerik.Web.UI.RadFileExplorer.prototype._showUploadValidationFailedAlert = function () {

    }

    function OnClientValidationFailed(sender, args) {
        var fileExtention = args.get_fileName().substring(args.get_fileName().lastIndexOf('.') + 1, args.get_fileName().length);
        if (args.get_fileName().lastIndexOf('.') != -1) {//this checks if the extension is correct
            if (sender.get_allowedFileExtensions().indexOf(fileExtention) == -1) {
                alert("Wrong Extension!");
            }
            else {
                alert("Wrong file size!");
            }
        }
        else {
            alert("not correct extension!");
        }
    }
</script>
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C#
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RadFileExplorer1.AsyncUpload.OnClientValidationFailed = "OnClientValidationFailed";
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VB
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RadFileExplorer1.AsyncUpload.OnClientValidationFailed = "OnClientValidationFailed"
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