expose common extensions for image/movie/audio types, since python scripts had to do this internally.

- bpy.path.extensions_image
- bpy.path.extensions_movie
- bpy.path.extensions_audio

eg:

  if os.path.splitext(filename)[1].lower() in bpy.path.extensions_image:
      ... we have an image ...

source/blender/python/generic/py_capi_utils.h

@@ -33,6 +33,7 @@ void			PyC_LineSpit(void);
 void			PyC_StackSpit(void);
 PyObject *		PyC_ExceptionBuffer(void);
 PyObject *		PyC_Object_GetAttrStringArgs(PyObject *o, Py_ssize_t n, ...);
+PyObject *		PyC_FrozenSetFromStrings(const char **strings);
 PyObject *		PyC_Err_Format_Prefix(PyObject *exception_type_prefix, const char *format, ...);
 void			PyC_FileAndNum(const char **filename, int *lineno);
 void			PyC_FileAndNum_Safe(const char **filename, int *lineno); /* checks python is running */