Per this post you will have to contact support requesting the change. https://confluence.atlassian.com/confkb/attachments-no-longer-open-in-browser-and-users-are-instead-prompted-to-download-the-files-779166001.html
Since I am a free plan user, Atlassian only gives me two support options on their "Contact Support" page:
Do you know any other contact method which I can use to request the change?
Thanks everyone for answering last week’s question. The winner of the random drawing from those who commented is: @LarryBrock I’ll contact you separately with your prize details. This wee...
Connect with like-minded Atlassian users at free events near you!Find an event
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no Community Events near you at the moment.Host an event
You're one step closer to meeting fellow Atlassian users at your local event. Learn more about Community Events