Bing found the following results
1 File - What is a .1 file and how do I open it? - FileInfo.com
Open files from the File menu - Microsoft Support
1 File: How to open 1 file (and what it is)
OpenTheFile - Open files with different extensions
How to Open and Extract .001 Files - Microsoft Community
3 Ways to Open a File in Windows - wikiHow
How to open files, folders, and apps in Windows 10 like a Pro
How To Open File With 1 Extension? - File Extension .1
1 file extension - What is a .1 format, and how to open it?
1 file extension - File-Extensions.org