diff --git a/rauth/session.py b/rauth/session.py
index 089854f..916f3d5 100644
--- a/rauth/session.py
+++ b/rauth/session.py
@@ -149,8 +149,9 @@ def request(self,
         url = self._set_url(url)
 
         entity_method = method.upper() in ENTITY_METHODS
-        if entity_method:
-            req_kwargs['headers'].setdefault('Content-Type', FORM_URLENCODED)
+        if entity_method and 'files' not in req_kwargs or \
+            req_kwargs['files'] is None:
+                req_kwargs['headers'].setdefault('Content-Type', FORM_URLENCODED)
 
         form_urlencoded = \
             req_kwargs['headers'].get('Content-Type') == FORM_URLENCODED