ok - revisiting this. It seems that some files saved as jpg in old posts can't be downloaded by clicking on them. I can use and FTP client and go to the "attachments" directory and retrieve them just fine.

here is a link with a "broken" example:
http://j30.us/ubbthreads/ubbthreads.php/topics/9649/1981_J_30_Hull_360_Hot_Chocola.html#Post9649
Note the two jpg files saved in the file manager. These were uploaded in June and worked fine until my upgrade to 7.5.6

here is a link with an uploaded jpg that works:
http://j30.us/ubbthreads/ubbthreads.php/topics/10120.html#Post10120
I just uploaded the jpg attachment for this test.

Try downloading the jpg in the 1st example and see what happens. This is driving me crazy!