I have many different types of files in a folder - .php, .css, .htm, .js, .docx, .xls, etc.
The only file type that Windows (viewed from Windows Explorer) actually shows the extension are the .php files.
Why is this?
Two things are most likely on your system for this to happen:
These combined will show .php file extensions while other documents are hidden.
You can change this when you open a folder and go to: Tools > Options > View
There you'll see the option to uncheck Hide extensions for known file types. Do so and save.
If you want to hide the PHP file extensions, leave the extensions option hidden and just register the filetype.
Look at one of the PHP file properties from explorer.
You seem to have your filename extensions visibility set to hidden.
You might have to change the properties of folders. By default windows hides extensions of known files.
Under Control Panel -> Folder option -> View tab, uncheck the "Hide extensions for known file types" box