To enable zooming on files, head to your workspace settings.
Go to the full screen image zoom section under the Files & Collections category, and toggle on Allow full-screen image zoom.
With zooming enabled, you'll now see a magnifying glass on all images in the single file view.
To zoom and open the file full-screen, click the magnifying glass. You can zoom in further with the + and - icons in the top right of the zoomed image