diff --git a/README.rst b/README.rst
index c9113b6..99c5c6a 100644
--- a/README.rst
+++ b/README.rst
@@ -134,10 +134,10 @@ Djrill supports most of the functionality of Django's `EmailMessage`_ and
   raise a ``djrill.NotSupportedByMandrillError`` exception when you attempt to
   send the message. (Mandrill doesn't support sending multiple html alternative
   parts, or any non-html alternatives.)
-* Djrill attempts to include a message's attachments, but Mandrill will
-  (silently) ignore any attachment types it doesn't allow. According to
-  Mandrill's docs, attachments are only allowed with the mimetypes "text/\*",
-  "image/\*", or "application/pdf".
+* Djrill includes a message's attachments, but only with the mimetypes "text/\*",
+  "image/\*", or "application/pdf" (since that is all Mandrill allows). Any
+  other attachment types will raise a ``djrill.NotSupportedByMandrillError``
+  exception when you attempt to send the message.
 * Djrill treats all cc and bcc recipients as if they were additional "to"
   addresses. (Mandrill does not distinguish cc, and only allows a single bcc --
   which Djrill doesn't use. *Caution:* depending on the ``preserve_recipients``
diff --git a/djrill/mail/backends/djrill.py b/djrill/mail/backends/djrill.py
index 73b8a42..98466a3 100644
--- a/djrill/mail/backends/djrill.py
+++ b/djrill/mail/backends/djrill.py
@@ -226,6 +226,18 @@ class DjrillBackend(BaseEmailBackend):
         if mimetype is None:
             mimetype = DEFAULT_ATTACHMENT_MIME_TYPE
 
+        # Mandrill silently filters attachments with unsupported mimetypes.
+        # This can be confusing, so we raise an exception instead.
+        (main, sub) = mimetype.lower().split('/')
+        attachment_allowed = (
+            main == 'text' or main == 'image'
+            or (main == 'application' and sub == 'pdf'))
+        if not attachment_allowed:
+            raise NotSupportedByMandrillError(
+                "Invalid attachment mimetype '%s'. Mandrill only supports "
+                "text/*, image/*, and application/pdf attachments."
+                % mimetype)
+
         return {
             'type': mimetype,
             'name': filename or "",
diff --git a/djrill/tests/test_mandrill_send.py b/djrill/tests/test_mandrill_send.py
index 2565ac1..2a86147 100644
--- a/djrill/tests/test_mandrill_send.py
+++ b/djrill/tests/test_mandrill_send.py
@@ -160,6 +160,24 @@ class DjrillBackendTests(DjrillBackendMockAPITestCase):
             msg="Mandrill API should not be called when send fails silently")
         self.assertEqual(sent, 0)
 
+    def test_attachment_errors(self):
+        # Mandrill silently strips attachments that aren't text/*, image/*,
+        # or application/pdf. We want to alert the Djrill user:
+        with self.assertRaises(NotSupportedByMandrillError):
+            msg = mail.EmailMessage('Subject', 'Body',
+                'from@example.com', ['to@example.com'])
+            # This is the default mimetype, but won't work with Mandrill:
+            msg.attach(content="test", mimetype="application/octet-stream")
+            msg.send()
+
+        with self.assertRaises(NotSupportedByMandrillError):
+            msg = mail.EmailMessage('Subject', 'Body',
+                'from@example.com', ['to@example.com'])
+            # Can't send Office docs, either:
+            msg.attach(filename="presentation.ppt", content="test",
+                mimetype="application/vnd.ms-powerpoint")
+            msg.send()
+
     def test_mandrill_api_failure(self):
         self.mock_post.return_value = self.MockResponse(status_code=400)
         with self.assertRaises(MandrillAPIError):