It is not a real intuitive process. First click the reply button then at the bottom of the reply box click "go advanced". Then below the advanced reply box there is a box called attachments, you click on the manage attachments box inside that box, a new screen opens that allows you to attach your files. I hope this helps, there are several other replies that may make the process clearer.
We will also need to know what .xdf file you are using, attach it also
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