diff --git a/CHANGES.rst b/CHANGES.rst
index a8d96a10de..1f175d84b5 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -23,6 +23,8 @@ Bugfixes
 - Fix unpickling of instances created before 4.0b2 those classes changed from
   old-style classes to new-style classes.
 
+- Prevent UnicodeDecodeError when publishing image (bytes) responses without content-type
+
 Changes
 +++++++
 
diff --git a/src/ZPublisher/HTTPResponse.py b/src/ZPublisher/HTTPResponse.py
index 96e536f406..7839a55d26 100644
--- a/src/ZPublisher/HTTPResponse.py
+++ b/src/ZPublisher/HTTPResponse.py
@@ -433,6 +433,10 @@ def insertBase(self):
                     self.setHeader('content-length', len(self.body))
 
     def isHTML(self, text):
+        try:
+            text = text.decode(self.charset)
+        except UnicodeDecodeError:
+            pass
         text = text.lstrip()
         # Note that the string can be big, so text.lower().startswith()
         # is more expensive than s[:n].lower().
@@ -514,7 +518,7 @@ def setBody(self, body, title='', is_error=False, lock=None):
         content_type = self.headers.get('content-type')
 
         if content_type is None:
-            if self.isHTML(body.decode(self.charset)):
+            if self.isHTML(body):
                 content_type = 'text/html; charset=%s' % self.charset
             else:
                 content_type = 'text/plain; charset=%s' % self.charset