In this dialog you can select one or more files from the ones already included in the project.

In the left pane you can see a tree containing all the folders of your project that contain at least one file or have been added explicitly to the project.

In the list from the right pane there are all the files from the selected folder in the left pane, that correspond to the filter from the “Files of type” field.

The “File name” field contains the name of the selected files - if there are any.