diff --git a/app/logical/media_file.rb b/app/logical/media_file.rb
index 534f91dbb..5ac19db15 100644
--- a/app/logical/media_file.rb
+++ b/app/logical/media_file.rb
@@ -60,10 +60,11 @@ class MediaFile
     # https://www.ftyps.com
     # isom (common) - MP4 Base Media v1 [IS0 14496-12:2003]
     # mp42 (common) - MP4 v2 [ISO 14496-14]
+    # iso5 (rare) - MP4 Base Media v5 (used by Twitter)
     # 3gp5 (rare) - 3GPP Media (.3GP) Release 5
     # avc1 (rare) - MP4 Base w/ AVC ext [ISO 14496-12:2005]
     # M4V (rare) - Apple iTunes Video (https://en.wikipedia.org/wiki/M4V)
-    when /\A....ftyp(?:isom|3gp5|mp42|avc1|M4V)/
+    when /\A....ftyp(?:isom|iso5|3gp5|mp42|avc1|M4V)/
       :mp4
     when /\APK\x03\x04/
       :zip
diff --git a/test/files/test-iso5.mp4 b/test/files/test-iso5.mp4
new file mode 100644
index 000000000..e3805f57e
Binary files /dev/null and b/test/files/test-iso5.mp4 differ
diff --git a/test/functional/uploads_controller_test.rb b/test/functional/uploads_controller_test.rb
index 0063f8959..c8ea90fa2 100644
--- a/test/functional/uploads_controller_test.rb
+++ b/test/functional/uploads_controller_test.rb
@@ -313,7 +313,8 @@ class UploadsControllerTest < ActionDispatch::IntegrationTest
         should_upload_successfully("https://pbs.twimg.com/media/DCdZ_FhUIAAYKFN?format=jpg&name=medium")
         should_upload_successfully("https://pbs.twimg.com/profile_banners/2371694594/1581832507/1500x500")
         should_upload_successfully("https://twitter.com/zeth_total/status/1355597580814585856")
-        # XXX should_upload_successfully("https://video.twimg.com/tweet_video/EWHWVrmVcAAp4Vw.mp4")
+        should_upload_successfully("https://video.twimg.com/tweet_video/FLKI6DWakAQFRkC.mp4")
+        should_upload_successfully("https://video.twimg.com/tweet_video/EWHWVrmVcAAp4Vw.mp4")
 
         should_upload_successfully("https://www.weibo.com/5501756072/J2UNKfbqV")
         should_upload_successfully("https://wx1.sinaimg.cn/mw690/0060kO5aly1gezsyt5xvhj30ok0sgtc9.jpg")
diff --git a/test/unit/media_file_test.rb b/test/unit/media_file_test.rb
index c1f4f9866..1f4b78194 100644
--- a/test/unit/media_file_test.rb
+++ b/test/unit/media_file_test.rb
@@ -97,6 +97,10 @@ class MediaFileTest < ActiveSupport::TestCase
       assert_equal(:mp4, MediaFile.open("test/files/test-audio.m4v").file_ext)
     end
 
+    should "determine the correct extension for an iso5 mp4 file" do
+      assert_equal(:mp4, MediaFile.open("test/files/test-iso5.mp4").file_ext)
+    end
+
     should "determine the correct extension for a ugoira file" do
       assert_equal(:zip, MediaFile.open("test/files/ugoira.zip").file_ext)
     end