Here is a post I found helpful for checking the Timeline Control settings on the form.

On the Timeline, there is a setting "Enable Attachment preview for file" make sure this is enabled so the viewing window appears.

If, you are experiencing issues with actually seeing information in the preview - ex: the preview appears blank with just the name of the file - try opening the record in an Incognito or different type of browser.

In Chrome, I was having trouble seeing the file preview, it just showed up as a blank screen with the file name. However, I could see the image attachments. When I switched to MS Edge, I was able to see both the PDF file and the PNG image.

Hope this helps!

Link to reference article: https://selfpacedcourses.softchief.com/how-to-enable-attachment-file-preview-in-power-apps-model-driven-app/