Consider the following scenario:
In Microsoft SharePoint Server 2013 or Microsoft SharePoint Online in Office 365, you change your search result types by specifying a display template other than the default template.
You perform a search and then view the results that are returned.
In this scenario, the document type icon isn't displayed, and the document preview doesn't work.
For document previews to work after templates are changed, you must follow these steps:
Create a new template for each different document search result type that you want to use.
In the Result Types section, and create a new copy of each affected result type that points to the new templates that you created.
This lets the search result Web Part select the correct template based on the kind of document. The Web Part will use templates that have previews for the file types that can display previews.
For more information about result types and templates, see About configuring result types.
This behavior is by design, depending on which template is used. For example, the Default Item template is usually used when a file of unknown type (such as source-code instead of a PowerPoint deck) is encountered. Therefore, this template doesn't show document previews.
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