Windows Explorer Preview Add text file type

Create a .reg file

paste this in to it:

Windows Registry Editor Version 5.00
[HKEY_CLASSES_ROOT\.properties]
@="properties_auto_file"
"Content Type"="text/plain"
"PerceivedType"="text"

change 'properties' online 3 and 4 to your file extension
(noting the use of '.')

save then double click the .reg file to add it to your registry.

you can now preview the file type in windows explorer's file preview panel.

(no need to restart / relog in or restart windows explorer.)