Thought I'd share how we're previewing files other than PDFs that the Knack viewer handles beautifully. One of our apps is dependent on file records and users need quick access to preview without downloading.
Importantly there's no file conversion performed which is problematic especially for Office documents where tables and text boxes typically don't behave with conversions we tried with cloud services. And Office documents get the benefit of the Microsoft Web Viewer menus and controls, including Excel table filters and Word accessibility features.
Check it out here https://support.knack.com/hc/en-us/community/posts/360072660492