In its event handler access the name and length (size) of the dragged file item and set them as values of two hidden fields on the page. Here you should also made an ajax request in which handler to update the content of the additional grid width these values.
I am attaching a basic implementation of the described approach to this message. Please, feel free to modify it in order to fit your scenario best. You can also find useful information about RadGrid row dragging in following demo: Grid - Items Drag-and-drop
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