Current behavior:

• One clic to select a document
• Second clic to select a second document (2 documents selected)
• Click on 'Select all' check box select all documents
• Second clic on 'Select all' check box deselect all documents

Commonly used behavior (Windows file system explorer, Google Drive, ...):

• One clic to select a document
• Second clic to select another document (only one document selected)
• Ctrl+clic to select more than one document
• Maj+clic is an other way to select several documents
• Clic outside deselect all documents

Windows file system explorer has also a 'Select all' button in the menu but it goes together with a 'None' button and a 'Reverse selection' button.

'None' seems more important than the 'Reverse'. It avoids to deselct all documents by clicking on 'Select all'...