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dhtmlxGrid threats content as HTML, so you can insert anything in a row.

grid.addRow(1,["<img src='some file'>"]);

or in case of using xml

<rows>
            <row>
                        <cell><![CDATA[ <img scr="some file " /> ]]></cell>

By default height of grid row is fixed, so you also probably will need to call

            grid.enableMultiline(true);

I hope this information will be enough for you. But you also can have a look at ajax library for php and php file upload progress bar.