diff --git a/docs/user/advanced.rst b/docs/user/advanced.rst
index 175fc9b7e5..62959a9367 100644
--- a/docs/user/advanced.rst
+++ b/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
diff --git a/docs/user/quickstart.rst b/docs/user/quickstart.rst
index 096693fbe5..5117ed0c86 100644
--- a/docs/user/quickstart.rst
+++ b/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
 ---------------------
diff --git a/requests/__init__.py b/requests/__init__.py
index 3d8188a707..f7924dc895 100644
--- a/requests/__init__.py
+++ b/requests/__init__.py
@@ -62,7 +62,8 @@
 from .status_codes import codes
 from .exceptions import (
     RequestException, Timeout, URLRequired,
-    TooManyRedirects, HTTPError, ConnectionError
+    TooManyRedirects, HTTPError, ConnectionError,
+    FileModeWarning,
 )
 
 # Set default logging handler to avoid "No handler found" warnings.
@@ -75,3 +76,8 @@ def emit(self, record):
             pass
 
 logging.getLogger(__name__).addHandler(NullHandler())
+
+import warnings
+
+# FileModeWarnings go off per the default.
+warnings.simplefilter('default', FileModeWarning, append=True)
diff --git a/requests/exceptions.py b/requests/exceptions.py
index 89135a802e..ba0b910e31 100644
--- a/requests/exceptions.py
+++ b/requests/exceptions.py
@@ -97,3 +97,18 @@ class StreamConsumedError(RequestException, TypeError):
 
 class RetryError(RequestException):
     """Custom retries logic failed"""
+
+
+# Warnings
+
+
+class RequestsWarning(Warning):
+    """Base warning for Requests."""
+    pass
+
+
+class FileModeWarning(RequestsWarning, DeprecationWarning):
+    """
+    A file was opened in text mode, but Requests determined its binary length.
+    """
+    pass
diff --git a/requests/utils.py b/requests/utils.py
index 4a8c6d7e06..5975d0f1f0 100644
--- a/requests/utils.py
+++ b/requests/utils.py
@@ -29,7 +29,7 @@
                      basestring)
 from .cookies import RequestsCookieJar, cookiejar_from_dict
 from .structures import CaseInsensitiveDict
-from .exceptions import InvalidURL
+from .exceptions import InvalidURL, FileModeWarning
 
 _hush_pyflakes = (RequestsCookieJar,)
 
@@ -60,7 +60,22 @@ def super_len(o):
         except io.UnsupportedOperation:
             pass
         else:
-            return os.fstat(fileno).st_size
+            filesize = os.fstat(fileno).st_size
+
+            # Having used fstat to determine the file length, we need to
+            # confirm that this file was opened up in binary mode.
+            if 'b' not in o.mode:
+                warnings.warn((
+                    "Requests has determined the content-length for this "
+                    "request using the binary size of the file: however, the "
+                    "file has been opened in text mode (i.e. without the 'b' "
+                    "flag in the mode). This may lead to an incorrect "
+                    "content-length. In Requests 3.0, support will be removed "
+                    "for files in text mode."),
+                    FileModeWarning
+                )
+
+            return filesize
 
     if hasattr(o, 'getvalue'):
         # e.g. BytesIO, cStringIO.StringIO