upload: Show text/plain uploads inline.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

zerver/lib/upload/base.py

@@ -22,6 +22,7 @@ INLINE_MIME_TYPES = [
     "image/jpeg",
     "image/png",
     "image/webp",
+    "text/plain",
     "video/mp4",
     "video/webm",
     # To avoid cross-site scripting attacks, DO NOT add types such

zerver/tests/test_upload.py

@@ -1005,14 +1005,11 @@ class FileUploadTest(UploadSerializeMixin, ZulipTestCase):
                 self.assertIn(content_disposition, response["Content-disposition"])
             self.assertEqual(set(response["Cache-Control"].split(", ")), {"private", "immutable"})
 
-        check_xsend_links(
-            "zulip.txt", "zulip.txt", 'filename="zulip.txt"', returned_attachment=True
-        )
+        check_xsend_links("zulip.txt", "zulip.txt", 'filename="zulip.txt"')
         check_xsend_links(
             "áéБД.txt",
             "%C3%A1%C3%A9%D0%91%D0%94.txt",
             "filename*=utf-8''%C3%A1%C3%A9%D0%91%D0%94.txt",
-            returned_attachment=True,
         )
         check_xsend_links(
             "zulip.html",