I am grateful for the help, and I suspect it is something to do with my computer. As yet I have no idea what it might be!
When I left-click the file-name above, it goes straight to a new pop-up window which says (at the top) "Enter name of file to save to ....", and at the bottom there's a pair of lines: the upper one has, pre-entered, the file name "attachment.php" and the lower one says "epub file (*.php)".
If I download this to the destination I select in the middle of the pop-up window, what I have is a file with a .php extension, which apparently nothing on my computer will touch.
If I right-click on it, I get the range of suggestions that Peter T has posted above, and a few others, but nothing useful. If I select "save link as", I get the same pop-up screen, with the same options, as I get if I left-click it.
It seems that all the books on the forum download as .php files for me. Obviously I haven't checked them all .... but a small sample gives the same result.
I really would appreciate a suggestion!