attachment: Add a nullable content-type.

This will be backfilled in a future commit, to prevent walking the
Attachments more than once.

zerver/migrations/0545_attachment_content_type.py

@@ -0,0 +1,20 @@
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("zerver", "0544_copy_avatar_images"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="archivedattachment",
+            name="content_type",
+            field=models.TextField(null=True),
+        ),
+        migrations.AddField(
+            model_name="attachment",
+            name="content_type",
+            field=models.TextField(null=True),
+        ),
+    ]

zerver/models/messages.py

@@ -683,6 +683,8 @@ class AbstractAttachment(models.Model):
     # Size of the uploaded file, in bytes
     size = models.IntegerField()
 
+    content_type = models.TextField(null=True)
+
     # The two fields below serve as caches to let us avoid looking up
     # the corresponding messages/streams to check permissions before
     # serving these files.