How can I preview Windows files without file extension?

According to all of above information, I have tested many times. we can rename the files without extension, no matter what other type of files you have in the folder.

1. Run cmd.exe as administrator.
2. Type CD C:\folderpath
3. Type rename *. *.txt instead of * *.txt
4. Now all the file without extension will become XXX.txt

If all the files has the extension, you will get The syntax of the command is incorrect.

Then you can use preview to see these files.

———————————————————————————————————————————— Update：

you can use the following command to rename all the files without extension in all subfolders.
for example:

CD:\abc   
for /r %d in (*.) do ren "%d" *.txt

Open the folder. Click on the empty area in address bar. Type cmd and press Enter. This should open the command prompt.

Type this command and confirm with Enter:

ren * *.txt

All files will have .txt appended to their names.