Disable feature added by recent version of OFS.Image which try to guess

the content_type by reading the id of object.
In ERP5, the content_type is reads from filename property thanks to 
interaction_workflow


git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@41123 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/ERP5/Document/File.py

@@ -128,6 +128,16 @@ class File(Document, CMFFile):
 
   getcontentlength = get_size
 
+  def _get_content_type(*args, **kw):
+    """Override original implementation from OFS/Image.py
+    to disable content_type discovery because
+    id of object its used to read the filename value.
+    In ERP5, an interaction
+    document_conversion_interaction_workflow/Document_file,
+    update the content_type by reading filename property
+    """
+    return None
+
   def _setFile(self, data, precondition=None):
     if data is not None and self.hasData() and \
       md5.md5(str(data.read())).digest() ==\