You can disable the Confluence Previews plugin under your System add-ons section. To do so you just need to go under your Manage add-ons page, select All add-ons on the drop box and then type Confluence Previews into the search field. Then disable it and the preview feature will not longer work.
I hope it helps!
Can you check the Confluence secure settings regarding attachments as per below KB:
In Confluence 5 series the files usually are downloaded instead of being previewed.
Hope it helps.
It's the main new feature of 5.7: https://confluence.atlassian.com/display/DOC/Confluence+5.7+Release+Notes
"The powerful new preview means you can view many file types right from your Confluence page, no need to download the file first."
Some of our users are getting confused, as when you click on the arrows on the left and right, it takes you to the next attachment (or previous version). They don't know to scroll up and down to get to the next page.
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