Fix documentation and add is_image_file (#507)

torchvision/datasets/folder.py

@@ -11,14 +11,27 @@ def has_file_allowed_extension(filename, extensions):
 
     Args:
         filename (string): path to a file
+        extensions (iterable of strings): extensions to consider (lowercase)
 
     Returns:
-        bool: True if the filename ends with a known image extension
+        bool: True if the filename ends with one of given extensions
     """
     filename_lower = filename.lower()
     return any(filename_lower.endswith(ext) for ext in extensions)
 
 
+def is_image_file(filename):
+    """Checks if a file is an allowed image extension.
+
+    Args:
+        filename (string): path to a file
+
+    Returns:
+        bool: True if the filename ends with a known image extension
+    """
+    return has_file_allowed_extension(filename, IMG_EXTENSIONS)
+
+
 def find_classes(dir):
     classes = [d for d in os.listdir(dir) if os.path.isdir(os.path.join(dir, d))]
     classes.sort()