len(data) 'Each part of the body must have a known size.' fix

Author: Dmitry V Selitsky

src/atom/http_core.py

@@ -115,7 +115,7 @@ def add_body_part(self, data, mime_type, size=None):
           size: int Required if the data is a file like object. If the data is a
                 string, the size is calculated so this parameter is ignored.
         """
-        if isinstance(data, str):
+        if hasattr(data, '__len__'):
             size = len(data)
         if size is None:
             # TODO: support chunked transfer if some of the body is of unknown size.