"An attachment cannot be directly added to a thread. It has to be added to a page. But you can reference the attachment in a thread by adding a link to the attachment.

This can be done by uploading the attachment to a page, then right clicking on the attachment file name. This will give you the opportunity to copy the link address of the attachment. That link (URL) can be pasted into a thread as the reference. Unfortunately the link is not clickable in the thread. It would have to copied and pasted into the browser window by the reader.
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