Mailgun: add AMPHTML support

CHANGELOG.rst

@@ -30,6 +30,12 @@ vNext
 
 *Unreleased changes*
 
+Features
+~~~~~~~~
+
+* **Mailgun:** Add support for AMP for Email
+  (via ``message.attach_alternative(..., "text/x-amp-html")``).
+
 Fixes
 ~~~~~
 

anymail/backends/mailgun.py

@@ -310,6 +310,14 @@ class MailgunPayload(RequestsPayload):
             self.unsupported_feature("multiple html parts")
         self.data["html"] = body
 
+    def add_alternative(self, content, mimetype):
+        if mimetype.lower() == "text/x-amp-html":
+            if "amp-html" in self.data:
+                self.unsupported_feature("multiple html parts")
+            self.data["amp-html"] = content
+        else:
+            super().add_alternative(content, mimetype)
+
     def add_attachment(self, attachment):
         # http://docs.python-requests.org/en/v2.4.3/user/advanced/#post-multiple-multipart-encoded-files
         if attachment.inline:

docs/esps/mailgun.rst

@@ -237,6 +237,13 @@ Limitations and quirks
   Your tracking webhooks will receive metadata values (either that you provided or the
   default empty string) for *every* key used with *any* recipient in the send.
 
+**AMP for Email**
+  Mailgun supports sending AMPHTML email content. To include it, use
+  ``message.attach_alternative("...AMPHTML content...", "text/x-amp-html")``
+  (and be sure to also include regular HTML and/or text bodies, too).
+
+  .. versionadded:: 8.2
+
 
 .. _undocumented API requirement:
     https://mailgun.uservoice.com/forums/156243-feature-requests/suggestions/35668606

tests/test_mailgun_backend.py

@@ -301,6 +301,15 @@ class MailgunBackendStandardEmailTests(MailgunBackendMockAPITestCase):
         with self.assertRaises(AnymailUnsupportedFeature):
             self.message.send()
 
+    def test_amp_html_alternative(self):
+        # Mailgun *does* support text/x-amp-html alongside text/html
+        self.message.attach_alternative("<p>HTML</p>", "text/html")
+        self.message.attach_alternative("<p>And AMP HTML</p>", "text/x-amp-html")
+        self.message.send()
+        data = self.get_api_call_data()
+        self.assertEqual(data["html"], "<p>HTML</p>")
+        self.assertEqual(data["amp-html"], "<p>And AMP HTML</p>")
+
     def test_alternatives_fail_silently(self):
         # Make sure fail_silently is respected
         self.message.attach_alternative("{'not': 'allowed'}", "application/json")