By default, Windows Explorer does not show filename extensions or hidden folders. You need to know what extensions files have, when, for example, you want to configure Sophos Anti Virus to exclude certain file types from scanning. You may also need to access hidden folders to do this.
The extension indicates what type of file it is. For example
<filename>.txt is a text file and
<filename>.exe is an executable file (i.e. one that runs a program).
This article is provided as a point of reference for existing Microsoft documentation.
What to do
Displaying file extensions
Displaying hidden folder
For other platforms and further information we recommend you consult Microsoft documentation.