guess attachment mimetype in Django email backend if not provided

* add test that demonstrates missing mimetype issue
* guess mimetype using python mimetypes lib if omitted
* add test with unguessable file type
* add fallback mimetype that django uses (application/octet-stream)

sparkpost/django/message.py

@@ -1,6 +1,8 @@
+import mimetypes
 from base64 import b64encode
 
 from django.core.mail import EmailMultiAlternatives
+from django.core.mail.message import DEFAULT_ATTACHMENT_MIME_TYPE
 from django.conf import settings
 
 from .exceptions import UnsupportedContent
@@ -63,6 +65,12 @@ def __init__(self, message):
             str_encoding = settings.DEFAULT_CHARSET
             for attachment in message.attachments:
                 filename, content, mimetype = attachment
+
+                if mimetype is None:
+                    mimetype, _ = mimetypes.guess_type(filename)
+                    if mimetype is None:
+                        mimetype = DEFAULT_ATTACHMENT_MIME_TYPE
+
                 try:
                     if isinstance(content, unicode):
                         content = content.encode(str_encoding)

test/django/test_message.py

@@ -108,6 +108,42 @@ def test_attachment_unicode():
     assert actual == expected
 
 
+def test_attachment_guess_mimetype():
+    email_message = EmailMessage(**base_options)
+    email_message.attach('file.txt', 'test content')
+
+    actual = SparkPostMessage(email_message)
+    expected = dict(
+        attachments=[
+            {
+                'name': 'file.txt',
+                'data': 'dGVzdCBjb250ZW50',
+                'type': 'text/plain'
+            }
+        ]
+    )
+    expected.update(base_expected)
+    assert actual == expected
+
+
+def test_attachment_guess_mimetype_fallback():
+    email_message = EmailMessage(**base_options)
+    email_message.attach('file', 'test content')
+
+    actual = SparkPostMessage(email_message)
+    expected = dict(
+        attachments=[
+            {
+                'name': 'file',
+                'data': 'dGVzdCBjb250ZW50',
+                'type': 'application/octet-stream'
+            }
+        ]
+    )
+    expected.update(base_expected)
+    assert actual == expected
+
+
 def test_content_subtype():
     email_message = EmailMessage(
         to=['to@example.com'],