I've been slow getting back on this, Sam, because my host had inadvertently installed the development version (yes, I know you write in red not to, but people who frequently install modules think they don't need to read the instructions.)

Now we have Stable R3.5 and the problem also affects messages without attachments.

In CONTRIB-2864 you wrote:
"Regarding the attachment bug, I just tested as follows:
1) create new forum, default settings
2) subscribe to it
3) add discussion, type some junk, select 'mail now', add an attachment (.csv file), post it
4) add a reply, type junk, 'mail now', attachment (same .csv file), post it
5) visit admin/cron.php to trigger email sending
note - my settings on this server are set to default html email

I received 2 emails, one corresponding to each post. Both emails contained correct links to the attachment. On clicking the link I was prompted to log in and then download the csv."

I followed your protocol except that I attached a .doc file as ordinary users would do.
I don't know how to "visit admin/cron.php to trigger email sending" but emails from the standard forums are coming through normally.

I use Stable R1.8 on two other Moodle platforms on MySql and it works perfectly.