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?
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