Inbound: improve inline content handling

* refactor: derive `AnymailInboundMessage` from `email.message.EmailMessage`
  rather than legacy Python 2.7 `email.message.Message`

* feat(inbound): replace confusing `inline_attachments` with `content_id_map`
  and `inlines`; rename `is_inline_attachment` to `is_inline`; deprecate old names 

Closes #328

---------

Co-authored-by: Mike Edmunds <medmunds@gmail.com>

tests/test_inbound.py

@@ -6,6 +6,7 @@ from textwrap import dedent
 from django.core.mail import SafeMIMEText
 from django.test import SimpleTestCase
 
+from anymail.exceptions import AnymailDeprecationWarning
 from anymail.inbound import AnymailInboundMessage
 
 from .utils import SAMPLE_IMAGE_FILENAME, sample_email_path, sample_image_content
@@ -200,7 +201,7 @@ class AnymailInboundMessageConstructionTests(SimpleTestCase):
             content_id="inline-id",
         )
         self.assertEqual(att.get_filename(), "Simulácia.txt")
-        self.assertTrue(att.is_inline_attachment())
+        self.assertTrue(att.is_inline())
         self.assertEqual(att.get_content_text(), "Unicode ✓")
 
     def test_parse_raw_mime(self):
@@ -446,7 +447,7 @@ class AnymailInboundMessageConveniencePropTests(SimpleTestCase):
         # Default empty list
         self.assertEqual(AnymailInboundMessage().attachments, [])
 
-    def test_inline_attachments_prop(self):
+    def test_content_id_map_prop(self):
         att = AnymailInboundMessage.construct_attachment(
             "image/png",
             SAMPLE_IMAGE_CONTENT,
@@ -455,10 +456,64 @@ class AnymailInboundMessageConveniencePropTests(SimpleTestCase):
         )
 
         msg = AnymailInboundMessage.construct(attachments=[att])
-        self.assertEqual(msg.inline_attachments, {"abc123": att})
+        self.assertEqual(msg.content_id_map, {"abc123": att})
+
+        with self.assertWarnsMessage(
+            AnymailDeprecationWarning,
+            "inline_attachments has been renamed to content_id_map and will be removed "
+            "in the near future.",
+        ):
+            self.assertEqual(msg.inline_attachments, {"abc123": att})
 
         # Default empty dict
-        self.assertEqual(AnymailInboundMessage().inline_attachments, {})
+        self.assertEqual(AnymailInboundMessage().content_id_map, {})
+
+    def test_inlines_prop(self):
+        raw = dedent(
+            """\
+            MIME-Version: 1.0
+            Subject: Message with inline parts
+            Content-Type: multipart/mixed; boundary="boundary-orig"
+
+            --boundary-orig
+            Content-Type: text/html; charset="UTF-8"
+
+            <img src="cid:abc123"> Here is a message!
+
+            --boundary-orig
+            Content-Type: image/png; name="sample_image.png"
+            Content-Disposition: inline
+            Content-ID: <abc123>
+            Content-Transfer-Encoding: base64
+
+            {image_content_base64}
+
+            --boundary-orig
+            Content-Type: image/png; name="sample_image_without_cid.png"
+            Content-Disposition: inline
+            Content-Transfer-Encoding: base64
+
+            {image_content_base64}
+
+            --boundary-orig--
+            """
+        ).format(image_content_base64=b64encode(SAMPLE_IMAGE_CONTENT).decode("ascii"))
+
+        msg = AnymailInboundMessage.parse_raw_mime(raw)
+        inlines = msg.inlines
+
+        self.assertEqual(len(inlines), 2)
+
+        self.assertEqual(inlines[0].get_content_type(), "image/png")
+        self.assertEqual(inlines[0].as_uploaded_file().name, "sample_image.png")
+
+        self.assertEqual(inlines[1].get_content_type(), "image/png")
+        self.assertEqual(
+            inlines[1].as_uploaded_file().name, "sample_image_without_cid.png"
+        )
+
+        self.assertEqual(len(msg.content_id_map.items()), 1)
+        self.assertIn("abc123", msg.content_id_map)
 
     def test_attachment_as_uploaded_file(self):
         raw = dedent(
@@ -609,9 +664,16 @@ class AnymailInboundMessageAttachedMessageTests(SimpleTestCase):
 
         orig_inline_att = orig_msg.get_payload(1)
         self.assertEqual(orig_inline_att.get_content_type(), "image/png")
-        self.assertTrue(orig_inline_att.is_inline_attachment())
+        self.assertTrue(orig_inline_att.is_inline())
         self.assertEqual(orig_inline_att.get_payload(decode=True), SAMPLE_IMAGE_CONTENT)
 
+        with self.assertWarnsMessage(
+            AnymailDeprecationWarning,
+            "is_inline_attachment has been renamed to is_inline and will be removed in "
+            "the near future.",
+        ):
+            self.assertTrue(orig_inline_att.is_inline_attachment())
+
     def test_construct_rfc822_attachment_from_data(self):
         # constructed message/rfc822 attachment should end up as parsed message
         # (same as if attachment was parsed from raw mime, as in previous test)