diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index 81d04531..19ab016a 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -30,6 +30,12 @@ vNext
 
 *Unreleased changes*
 
+Fixes
+~~~~~
+
+* **Postmark:** Workaround for handling inbound test webhooks.
+  (`More info <https://github.com/anymail/django-anymail/issues/304>`__)
+
 Other
 ~~~~~
 
diff --git a/anymail/webhooks/postmark.py b/anymail/webhooks/postmark.py
index 4b13aed2..e6824b92 100644
--- a/anymail/webhooks/postmark.py
+++ b/anymail/webhooks/postmark.py
@@ -169,7 +169,13 @@ def esp_to_anymail_event(self, esp_event):
         attachments = [
             AnymailInboundMessage.construct_attachment(
                 content_type=attachment["ContentType"],
-                content=attachment["Content"],
+                content=(
+                    attachment.get("Content")
+                    # WORKAROUND:
+                    # The test webhooks are not like their real webhooks
+                    # This allows the test webhooks to be parsed.
+                    or attachment["Data"]
+                ),
                 base64=True,
                 filename=attachment.get("Name", "") or None,
                 content_id=attachment.get("ContentID", "") or None,
diff --git a/tests/test_postmark_inbound.py b/tests/test_postmark_inbound.py
index 18fa3957..47a92b22 100644
--- a/tests/test_postmark_inbound.py
+++ b/tests/test_postmark_inbound.py
@@ -165,6 +165,16 @@ def test_attachments(self):
                     "ContentType": 'message/rfc822; charset="us-ascii"',
                     "ContentLength": len(email_content),
                 },
+                # This is an attachement like send by the test webhook
+                # A workaround is implemented to handle it.
+                # Once Postmark solves the bug on their side this workaround
+                # can be reverted.
+                {
+                    "Name": "test.txt",
+                    "ContentType": "text/plain",
+                    "Data": "VGhpcyBpcyBhdHRhY2htZW50IGNvbnRlbnRzLCBiYXNlLTY0IGVuY29kZWQu",
+                    "ContentLength": 45,
+                },
             ]
         }
 
@@ -183,7 +193,7 @@ def test_attachments(self):
         event = kwargs["event"]
         message = event.message
         attachments = message.attachments  # AnymailInboundMessage convenience accessor
-        self.assertEqual(len(attachments), 2)
+        self.assertEqual(len(attachments), 3)
         self.assertEqual(attachments[0].get_filename(), "test.txt")
         self.assertEqual(attachments[0].get_content_type(), "text/plain")
         self.assertEqual(attachments[0].get_content_text(), "test attachment")
@@ -192,6 +202,14 @@ def test_attachments(self):
             attachments[1].get_content_bytes(), email_content
         )
 
+        # Attachment of test webhook
+        self.assertEqual(attachments[2].get_filename(), "test.txt")
+        self.assertEqual(attachments[2].get_content_type(), "text/plain")
+        self.assertEqual(
+            attachments[2].get_content_text(),
+            "This is attachment contents, base-64 encoded.",
+        )
+
         inlines = message.inline_attachments
         self.assertEqual(len(inlines), 1)
         inline = inlines["abc123"]