Unfortunately, there are not really written instructions at the top of the screen telling you exactly what to do, so here's the full procedure:

Firstly, prepare a sample Excel workbook, which should reflect the variations in your data and show how it is laid out, and should show a few examples of what you want to achieve.

Then you can click on Go Advanced (below the Edit Window) while you are composing a reply, then scroll down to and click on Manage Attachments and the Upload window will open. Click on Browse and navigate to (and double-click) the file icon that you want to attach, then click on Upload and then on Close this Window to return to the Edit window. When you have finished composing your post, click on Submit Post.

Don't try to use the Paperclip icon, as it doesn't work on this forum.

Hope this helps.

Pete