Merge pull request #3074 from tomchristie/unsaved_file_objects_serialize_to_none

Unsaved file objects may not have an associated URL.

rest_framework/fields.py

@@ -1198,9 +1198,13 @@ class FileField(Field):
         return data
 
     def to_representation(self, value):
-        if self.use_url:
         if not value:
             return None
+
+        if self.use_url:
+            if not getattr(value, 'url', None):
+                # If the file has not been saved it may not have a URL.
+                return None
             url = value.url
             request = self.context.get('request', None)
             if request is not None: