Perhaps the problem is your preferences are not possible. Delete any entry in the attachment settings for the file type in question, restart Thunderbird and try again.

kkepler said

Have all settings in options set to my preferences ...clicking on attachment does not bring up a dialog box for open or save, nothing happens...right click for options, choose open...again, nothing happens. All I can do with attachment is save it. No error messages appear.

Thank you so much! So simple too :)