If a certain file type, such as .png, does not appear in search results, you can add the file type to the Search Service Application using the SharePoint Central Administrator.
To add the file type extension to Search Service Application:
Application Management > Search Service Application > File Type on the left side under
Crawling , choose
SharePoint Central Admin page > Manage Service Application .
- Add the file type.
- Perform a full search crawl. The new file types show up in search results after the crawl.