diff --git a/README.md b/README.md
index 7fb4c3ea80..97c8e390be 100644
--- a/README.md
+++ b/README.md
@@ -173,7 +173,7 @@ dataset:
   normal_test_dir: null # name of the folder containing normal test images.
   task: segmentation # classification or segmentation
   mask: <path/to/mask/annotations> #optional
-  extensions: null
+  extensions: null # .ext or [.ext1, .ext2, ...]
   split_ratio: 0.2 # ratio of the normal images that will be used to create a test split
   image_size: 256
   train_batch_size: 32
diff --git a/src/anomalib/data/utils/path.py b/src/anomalib/data/utils/path.py
index 93784ba57b..663e201243 100644
--- a/src/anomalib/data/utils/path.py
+++ b/src/anomalib/data/utils/path.py
@@ -58,13 +58,16 @@ def _prepare_files_labels(
     if isinstance(extensions, str):
         extensions = (extensions,)
 
+    if not all(extension.startswith(".") for extension in extensions):
+        raise RuntimeError(f"All extensions {extensions} must start with the dot")
+
     filenames = [
         f
         for f in path.glob("**/*")
         if f.suffix in extensions and not f.is_dir() and not any(part.startswith(".") for part in f.parts)
     ]
     if not filenames:
-        raise RuntimeError(f"Found 0 {path_type} images in {path}")
+        raise RuntimeError(f"Found 0 {path_type} images in {path} with extensions {extensions}")
 
     labels = [path_type] * len(filenames)