Mailjet: Prevent empty attachment filename

Mailjet requires all attachments/inlines have a non-empty Filename field.
Substitute `"attachment"` for missing filenames.

Fixes #407.

CHANGELOG.rst

@@ -37,6 +37,13 @@ Breaking changes
   (Postal's signature verification uses the "cryptography" package, which is no
   longer reliably installable with Python 3.8.)
 
+Fixes
+~~~~~
+
+* **Mailjet:** Avoid a Mailjet API error when sending an inline image without a
+  filename. (Anymail now substitutes ``"attachment"`` for the missing filename.)
+  (Thanks to `@chickahoona`_ for reporting the issue.)
+
 
 v12.0
 -----
@@ -1737,6 +1744,7 @@ Features
 .. _@b0d0nne11: https://github.com/b0d0nne11
 .. _@calvin: https://github.com/calvin
 .. _@carrerasrodrigo: https://github.com/carrerasrodrigo
+.. _@chickahoona: https://github.com/chickahoona
 .. _@chrisgrande: https://github.com/chrisgrande
 .. _@cjsoftuk: https://github.com/cjsoftuk
 .. _@costela: https://github.com/costela

anymail/backends/mailjet.py

@@ -195,7 +195,8 @@ def set_html_body(self, body):
     def add_attachment(self, attachment):
         att = {
             "ContentType": attachment.mimetype,
-            "Filename": attachment.name or "",
+            # Mailjet requires a non-empty Filename.
+            "Filename": attachment.name or "attachment",
             "Base64Content": attachment.b64content,
         }
         if attachment.inline:

docs/esps/mailjet.rst

@@ -144,6 +144,15 @@ Limitations and quirks
   :ref:`esp-send-status`, because Mailjet's other (statistics, event tracking)
   APIs don't yet support MessageUUID.
 
+**Attachments require filenames**
+  Mailjet requires that all attachments and inline images have filenames. If you
+  don't supply a filename, Anymail will use ``"attachment"`` as the filename.
+
+  .. versionchanged:: 13.0
+
+     Earlier Anymail versions would default to an empty string, resulting in
+     a Mailjet API error.
+
 **Older limitations**
 
 .. versionchanged:: 6.0

tests/test_mailjet_backend.py

@@ -13,7 +13,7 @@
     AnymailSerializationError,
     AnymailUnsupportedFeature,
 )
-from anymail.message import attach_inline_image_file
+from anymail.message import attach_inline_image, attach_inline_image_file
 
 from .mock_requests_backend import (
     RequestsBackendMockAPITestCase,
@@ -266,7 +266,7 @@ def test_attachments(self):
         self.assertNotIn("ContentID", attachments[1])
 
         self.assertEqual(attachments[2]["ContentType"], "application/pdf")
-        self.assertEqual(attachments[2]["Filename"], "")  # none
+        self.assertEqual(attachments[2]["Filename"], "attachment")
         self.assertEqual(decode_att(attachments[2]["Base64Content"]), pdf_content)
         self.assertNotIn("ContentID", attachments[2])
 
@@ -297,6 +297,7 @@ def test_embedded_images(self):
         image_data = sample_image_content(image_filename)
 
         cid = attach_inline_image_file(self.message, image_path)  # Read from a png file
+        cid2 = attach_inline_image(self.message, image_data)
         html_content = (
             '<p>This has an <img src="cid:%s" alt="inline" /> image.</p>' % cid
         )
@@ -307,11 +308,16 @@ def test_embedded_images(self):
         self.assertEqual(data["Globals"]["HTMLPart"], html_content)
 
         attachments = data["Globals"]["InlinedAttachments"]
-        self.assertEqual(len(attachments), 1)
+        self.assertEqual(len(attachments), 2)
         self.assertEqual(attachments[0]["Filename"], image_filename)
         self.assertEqual(attachments[0]["ContentID"], cid)
         self.assertEqual(attachments[0]["ContentType"], "image/png")
         self.assertEqual(decode_att(attachments[0]["Base64Content"]), image_data)
+        # Mailjet requires a filename for all attachments, so make sure it's not empty:
+        self.assertEqual(attachments[1]["Filename"], "attachment")
+        self.assertEqual(attachments[1]["ContentID"], cid2)
+        self.assertEqual(attachments[1]["ContentType"], "image/png")
+        self.assertEqual(decode_att(attachments[1]["Base64Content"]), image_data)
 
         self.assertNotIn("Attachments", data["Globals"])
 
@@ -340,7 +346,7 @@ def test_attached_images(self):
                     "Base64Content": image_data_b64,
                 },
                 {
-                    "Filename": "",  # the unnamed one
+                    "Filename": "attachment",  # the unnamed one
                     "ContentType": "image/png",
                     "Base64Content": image_data_b64,
                 },

tests/test_mailjet_integration.py

@@ -7,7 +7,7 @@
 from anymail.exceptions import AnymailAPIError
 from anymail.message import AnymailMessage
 
-from .utils import AnymailTestMixin, sample_image_path
+from .utils import AnymailTestMixin, sample_image_content
 
 ANYMAIL_TEST_MAILJET_API_KEY = os.getenv("ANYMAIL_TEST_MAILJET_API_KEY")
 ANYMAIL_TEST_MAILJET_SECRET_KEY = os.getenv("ANYMAIL_TEST_MAILJET_SECRET_KEY")
@@ -91,7 +91,7 @@ def test_all_options(self):
         )
         message.attach("attachment1.txt", "Here is some\ntext for you", "text/plain")
         message.attach("attachment2.csv", "ID,Name\n1,Amy Lina", "text/csv")
-        cid = message.attach_inline_image_file(sample_image_path())
+        cid = message.attach_inline_image(sample_image_content())
         message.attach_alternative(
             "<p><b>HTML:</b> with <a href='http://example.com'>link</a>"
             "and image: <img src='cid:%s'></div>" % cid,