When the max number of attachments is set to zero or to more than one, there is no ambiguity. The ambiguity arises when that max number is set to 1.
Ideally a different online help message should be displayed in that case, stating "You can optionally attach one file to a forum post. If you attach an image, it will be displayed after the message."
A compromise solution which would not involve any re-writing of the Moodle core would be to re-phrase the help message like "You can optionally attach one or more files to a forum post (depending on this forum's settings). If you attach an image, it will be displayed after the message."
What do people think?