How to modify QuickView header icon?
Customizing icons in result templates does not customizes icon in QuickView header accordingly.
The icon is resolved depending on the FileType, and not the icon used in the current template.
Here is the FileTypes class that resolves this type.
You could have a look at the CSS class generated for this icon and see if you can override the image with your own.
Opened enhancement suggestion: https://coveo.ideas.aha.io/ideas/CID-I-428