diff --git a/torchvision/datasets/folder.py b/torchvision/datasets/folder.py
index bfe8e1ba5eb..ca103080ecf 100644
--- a/torchvision/datasets/folder.py
+++ b/torchvision/datasets/folder.py
@@ -4,14 +4,20 @@
 import os
 import os.path
 
-IMG_EXTENSIONS = [
-    '.jpg', '.JPG', '.jpeg', '.JPEG',
-    '.png', '.PNG', '.ppm', '.PPM', '.bmp', '.BMP',
-]
+IMG_EXTENSIONS = ['.jpg', '.jpeg', '.png', '.ppm', '.bmp', '.pgm']
 
 
 def is_image_file(filename):
-    return any(filename.endswith(extension) for extension in IMG_EXTENSIONS)
+    """Checks if a file is an image.
+
+    Args:
+        filename (string): path to a file
+
+    Returns:
+        bool: True if the filename ends with a known image extension
+    """
+    filename_lower = filename.lower()
+    return any(filename_lower.endswith(ext) for ext in IMG_EXTENSIONS)
 
 
 def find_classes(dir):