intf: WebVTT extensions detection

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

include/vlc_interface.h

@@ -261,7 +261,8 @@ typedef enum vlc_dialog {
                             "*.jss;*.psb;" \
                             "*.rt;*.smi;*.txt;" \
                             "*.smil;*.stl;*.usf;" \
-                            "*.dks;*.pjs;*.mpl2;*.mks"
+                            "*.dks;*.pjs;*.mpl2;*.mks;" \
+                            "*.vtt"
 
 /** \defgroup vlc_interaction Interaction
  * \ingroup vlc_interface