Raise ValueError for io.StringIO in Image.open

Tests/test_image.py

@@ -1,3 +1,4 @@
+import io
 import os
 import shutil
 import tempfile
@@ -91,6 +92,9 @@ def test_invalid_image(self):
     def test_bad_mode(self):
         self.assertRaises(ValueError, Image.open, "filename", "bad mode")
 
+    def test_stringio(self):
+        self.assertRaises(ValueError, Image.open, io.StringIO())
+
     def test_pathlib(self):
         from PIL.Image import Path
 

src/PIL/Image.py

@@ -2690,10 +2690,17 @@ def open(fp, mode="r"):
     :exception FileNotFoundError: If the file cannot be found.
     :exception PIL.UnidentifiedImageError: If the image cannot be opened and
        identified.
+    :exception ValueError: If the ``mode`` is not "r", or if a ``StringIO``
+       instance is used for ``fp``.
     """
 
     if mode != "r":
         raise ValueError("bad mode %r" % mode)
+    elif isinstance(fp, io.StringIO):
+        raise ValueError(
+            "StringIO cannot be used to open an image. "
+            "Binary data must be used instead."
+        )
 
     exclusive_fp = False
     filename = ""