When trying to drag files with not-so-common extensions to upload, they are not uploaded.

For the time being, you can workaround the issue by using this patch:

<telerik:RadFileExplorer runat="server" ID="RFE1">
    <Configuration ViewPaths="~/" UploadPaths="~/" DeletePaths="~/" SearchPatterns="*.*"/>
</telerik:RadFileExplorer>


<script>
    Telerik.Web.UI.RadFileExplorer.prototype._onAsyncUploadFilesUploading = function (sender, args) {
        var currentDroppedFileType = this._pullNextDroppedFileType();
        if (this._isDropUploadCancelled === true) {
            args.set_cancel(true);
            if (this._isThisLastUploadingFile()) this._finalizeFileDrop();
        }
        this._currentUploadRow = args.get_row();

        if (this._btnUpload != null) {
            this._btnUpload.control.set_enabled(false);
        }
    };
</script>