Hi, I'm Bhavik. Welcome to JustAnswer. I'm reviewing your question now, and will reply back ASAP.
Which attachments you are trying to open, Does this contains PDF, ZIP files or any specific files etc ? For which application you are facing this issue ?
I can see many mac user have faced this issue with outlook. Are you also facing this issue with outlook ?
Are the existing saved PDF files are opening normally ? If so then we will have to fix the problem associated with browse's settings.