GtkFileChooser: How to select files that are inside subfolders?

[Phong Cao <phngcv gmail com>]

Hello everybody,

I am trying to develop a music manager that lets the users import all audio files from a folder. That folder may contain many subfolders, and each subfolder may contain many files. I know that I need to create a GtkFileChooser Dialog with GTK_FILE_CHOOSER_ACTION_SELECT_FOLDER enabled so that the user can choose folders instead of files. 

However, I do not know how to make this GtkFileChooser selects/imports all the files available in that directory (not only the files that are shown in the GtkFileChooser but the files that are deeply contained inside subfolders).

Here is a screenshot of the application. How can I make GtkFileChooser imports all the files that are deeply contained inside this "firefox" folder?

Thank you for reading my questions! I hope you guys have a good day!