Several gnome extensions like -documents, -music and -photos rely on tracker to find and index document, music and photo formats.
In order that this works properly, the xdg-user-dirs extension can be used to set up various xdg stuff before starting gnome-session: $ xdg-user-dirs-update
..and then, after starting gnome-session, you can check things are working properly by, for example: $ tracker search -m
Files:
file:///home/tc/Music/aayw.mp3
By default tracker will only look in the xdg user dirs, but other file locations can be added manually.