I have added content search web part to the site page. And it is showing me the contents that are the word documents. But it is not showing me the notepad files that I have added to the document library. Why does that happen so? Is that the thing that whenever we go to add a new file instead of upload SharePoint does not show the notepad template, that's what the reason. Or is that it only shows the Microsoft Word, Excel, PowerPoint & OneNote.
It is possible that your Search Service needs to re-crawl the library before the CSWP will display the new results.
You may want to also check to see if "txt" is an accepted filetype for Search. By default, it is.