attachments: Return a list of removed attachments while editing.

Currently, we want to ask users if they would like to delete their
attachments after they have removed the attachments while editing. These
changes are preparatory changes on the backend to return a list of removed
attachments after the user has removed attachments while editing.

Fixes part of #25525.

zerver/lib/test_classes.py

@@ -16,6 +16,7 @@ import orjson
 import responses
 from django.apps import apps
 from django.conf import settings
+from django.core.files.uploadedfile import UploadedFile
 from django.core.mail import EmailMessage
 from django.core.signals import got_request_exception
 from django.db import connection, transaction
@@ -76,6 +77,7 @@ from zerver.lib.test_helpers import (
 )
 from zerver.lib.thumbnail import ThumbnailFormat
 from zerver.lib.topic import RESOLVED_TOPIC_PREFIX, filter_by_topic_name_via_message
+from zerver.lib.upload import upload_message_attachment_from_request
 from zerver.lib.user_groups import get_system_user_group_for_user
 from zerver.lib.users import get_api_key
 from zerver.lib.webhooks.common import (
@@ -2027,6 +2029,14 @@ Output:
         ):
             yield
 
+    def create_attachment_helper(self, user: UserProfile) -> str:
+        with tempfile.NamedTemporaryFile() as attach_file:
+            attach_file.write(b"Hello, World!")
+            attach_file.flush()
+            with open(attach_file.name, "rb") as fp:
+                file_path = upload_message_attachment_from_request(UploadedFile(fp), user)
+                return file_path
+
 
 class ZulipTestCase(ZulipTestCaseMixin, TestCase):
     @contextmanager