An often asked question is to have Document Search, where the want is to list documents only - typically Office type formats like Word, Excel and PowerPoint. But the exact answer is not that white/black, and can differ from company to company.
One query you might consider executing to list documents only in SharePoint is IsDocument:true.
This query will return everything SharePoint deems a file or web page.. or not a list item (almost), returning Office documents, images, zip files… you name it, it will be part of the returned result set.
Which brings up the question: What is a document?