diff --git a/ckanext/big_resources/plugin.py b/ckanext/big_resources/plugin.py
index 6839c19..57feccf 100644
--- a/ckanext/big_resources/plugin.py
+++ b/ckanext/big_resources/plugin.py
@@ -34,7 +34,7 @@ def _copy_file_overwriten(input_file, output_file, max_size):
     CHUNK_SIZE = int(toolkit.config.get('ckanext.big_resources.chunk_upload', 2048))
     
     max_size_in_mb = max_size*1000000
-    if len(input_file.read()) > max_size_in_mb:
+    if os.path.getsize(input_file.name) > max_size_in_mb:
         raise logic.ValidationError({'upload': ['File upload too large']})
     input_file.seek(0)
     while True: