There are two ways to select all files inside a folder:
1. Using the File List:
Right click and choose "Select/Deselect All".
2. Using the Folder Tree:
Left click on the folder picture of the folder you want to select/deselect. Note that if the folder you are clicking on has a plus sign (+) to the right of the folder picture, you will also select/deselect all subfolders too by this method.