diff --git a/sendgrid/message.py b/sendgrid/message.py
index <HASH>..<HASH> 100644
--- a/sendgrid/message.py
+++ b/sendgrid/message.py
@@ -128,11 +128,9 @@ class Mail(SMTPAPIHeader):
     def add_attachment_stream(self, name, string):

         if sys.version_info < (3, 0) and isinstance(name, unicode):

             name = name.encode('utf-8')

-        if isinstance(string, str):

-            self.files[name] = string

-        elif isinstance(string, io.BytesIO):

+        if isinstance(string, io.BytesIO):

             self.files[name] = string.read()

-        elif sys.version_info < (3, 0) and isinstance(string, unicode):

+        else:

             self.files[name] = string

 

     def add_content_id(self, cid, value):
Small conditional clean-up

The conditional under revision does the same thing for two of three cases. I just got
rid of their separation.