Add warnings about text/binary mode files.

docs/user/advanced.rst

@@ -315,6 +315,15 @@ file-like object for your body::
     with open('massive-body', 'rb') as f:
         requests.post('http://some.url/streamed', data=f)
 
+.. warning:: It is strongly recommended that you open files in `binary mode`_.
+             This is because Requests may attempt to provide the
+             ``Content-Length`` header for you, and if it does this value will
+             be set to the number of *bytes* in the file. Errors may occur if
+             you open the file in *text mode*.
+
+.. _binary mode: https://docs.python.org/2/tutorial/inputoutput.html#reading-and-writing-files
+
+
 .. _chunk-encoding:
 
 Chunk-Encoded Requests
@@ -362,6 +371,15 @@ To do that, just set files to a list of tuples of (form_field_name, file_info):
       ...
     }
 
+.. warning:: It is strongly recommended that you open files in `binary mode`_.
+             This is because Requests may attempt to provide the
+             ``Content-Length`` header for you, and if it does this value will
+             be set to the number of *bytes* in the file. Errors may occur if
+             you open the file in *text mode*.
+
+.. _binary mode: https://docs.python.org/2/tutorial/inputoutput.html#reading-and-writing-files
+
+
 .. _event-hooks:
 
 Event Hooks
@@ -519,7 +537,7 @@ any request to the given scheme and exact hostname.
     }
 
 Note that proxy URLs must include the scheme.
-    
+
 .. _compliance:
 
 Compliance

docs/user/quickstart.rst

@@ -302,6 +302,14 @@ support this, but there is a separate package which does -
 For sending multiple files in one request refer to the :ref:`advanced <advanced>`
 section.
 
+.. warning:: It is strongly recommended that you open files in `binary mode`_.
+             This is because Requests may attempt to provide the
+             ``Content-Length`` header for you, and if it does this value will
+             be set to the number of *bytes* in the file. Errors may occur if
+             you open the file in *text mode*.
+
+.. _binary mode: https://docs.python.org/2/tutorial/inputoutput.html#reading-and-writing-files
+
 
 Response Status Codes
 ---------------------