Add support for managing and deleting attachments.

Modified substantially by tabbott to fix tons of issues.

Fixes #454.

zerver/lib/attachments.py

@@ -0,0 +1,29 @@
+from __future__ import absolute_import
+
+from django.utils.translation import ugettext as _
+from typing import Any, Dict, List
+
+from zerver.lib.request import JsonableError
+from zerver.lib.upload import delete_message_image
+from zerver.models import Attachment, UserProfile
+
+def user_attachments(user_profile):
+    # type: (UserProfile) -> List[Dict[str, Any]]
+    attachments = Attachment.objects.filter(owner=user_profile).prefetch_related('messages')
+    return [a.to_dict() for a in attachments]
+
+def access_attachment_by_id(user_profile, attachment_id, needs_owner=False):
+    # type: (UserProfile, int, bool) -> Attachment
+    query = Attachment.objects.filter(id=attachment_id)
+    if needs_owner:
+        query = query.filter(owner=user_profile)
+
+    attachment = query.first()
+    if attachment is None:
+        raise JsonableError(_("Invalid attachment"))
+    return attachment
+
+def remove_attachment(user_profile, attachment):
+    # type: (UserProfile, Attachment) -> None
+    delete_message_image(attachment.path_id)
+    attachment.delete()