On the Actions menu, the Exclude command filters out the selected files and folders from the current session. It is a handy alternative to opening the File Filters dialog and entering file/folder exclusions.
If you select one or more files of the same type, the Exclude command will offer to exclude all files of that type.
To remove an exclude, click the file filters icon 
and remove the entry from the Exclude files (or Exclude folders) list.