If you do this from the Windows tool (not BIOS!!!), the data cached in the Optane memory will be written to the HDD and the HDD will not lose anything.
To attach file, use the Drag and drop here or browse files to attach dialog (below the edit box for your response post). Simply drag a file from file explorer to this dialog and it will be attached to your response post.
As for including pictures, etc. right in the post, as Al says, simply cut-and-paste or copy-and-paste.