diff --git a/requests/__init__.py b/requests/__init__.py
index 3d8188a7..f7924dc8 100644
--- a/requests/__init__.py
+++ b/requests/__init__.py
@@ -62,7 +62,8 @@ from .sessions import session, Session
 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 @@ except ImportError:
             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 89135a80..ba0b910e 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 4a8c6d7e..5975d0f1 100644
--- a/requests/utils.py
+++ b/requests/utils.py
@@ -29,7 +29,7 @@ from .compat import (quote, urlparse, bytes, str, OrderedDict, unquote, is_py2,
                      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