Since version 4.1.14, CJ Duplicate Member Logger is preventing any attached file from being downloaded at our install.
This is the answer we got from IPS support:
This is actually due to the "CJ Duplicate Member Logger" application that is currently installed. If I disable that application, then I am able to download the attachment normally.
I would recommend contacting the author of that application regarding this issue.
Invision Power Services, Inc.