diff --git a/app/logical/media_file.rb b/app/logical/media_file.rb
index 9b74ef336..4c87a9fce 100644
--- a/app/logical/media_file.rb
+++ b/app/logical/media_file.rb
@@ -67,14 +67,20 @@ class MediaFile
       :webp
 
     # https://www.ftyps.com
+    # https://cconcolato.github.io/mp4ra/filetype.html
+    # https://github.com/mozilla/gecko-dev/blob/master/toolkit/components/mediasniffer/nsMediaSniffer.cpp#L78
+    # https://mimesniff.spec.whatwg.org/#signature-for-mp4
+    #
     # isom (common) - MP4 Base Media v1 [IS0 14496-12:2003]
     # mp42 (common) - MP4 v2 [ISO 14496-14]
+    # iso4 (rare) - MP4 Base Media v4
     # iso5 (rare) - MP4 Base Media v5 (used by Twitter)
-    # 3gp5 (rare) - 3GPP Media (.3GP) Release 5
+    # 3gp5 (rare) - 3GPP Media (.3GP) Release 5 (XXX technically this should be .3gp, not .mp4. Supported by Chrome but not Firefox)
     # 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|iso5|3gp5|mp42|avc1|M4V)/
+    when /\A....ftyp(?:mp4|avc|iso|3gp5|M4V)/
       :mp4
+
     # https://aomediacodec.github.io/av1-avif/#brands-overview
     when /\A....ftyp(?:avif|avis)/
       :avif
diff --git a/test/files/mp4/test-300x300-3gp5.mp4 b/test/files/mp4/test-300x300-3gp5.mp4
new file mode 100644
index 000000000..e94c1b75c
Binary files /dev/null and b/test/files/mp4/test-300x300-3gp5.mp4 differ
diff --git a/test/files/mp4/test-300x300-iso4.mp4 b/test/files/mp4/test-300x300-iso4.mp4
new file mode 100644
index 000000000..c0f354d3c
Binary files /dev/null and b/test/files/mp4/test-300x300-iso4.mp4 differ
diff --git a/test/functional/uploads_controller_test.rb b/test/functional/uploads_controller_test.rb
index d480ec790..c000b65d1 100644
--- a/test/functional/uploads_controller_test.rb
+++ b/test/functional/uploads_controller_test.rb
@@ -372,8 +372,10 @@ class UploadsControllerTest < ActionDispatch::IntegrationTest
         should_upload_successfully("test/files/mp4/test-300x300.mp4")
         should_upload_successfully("test/files/mp4/test-300x300-vp9.mp4")
         should_upload_successfully("test/files/mp4/test-300x300-yuvj420p-h264.mp4")
+        should_upload_successfully("test/files/mp4/test-300x300-iso4.mp4")
         should_upload_successfully("test/files/mp4/test-audio.mp4")
         should_upload_successfully("test/files/mp4/test-audio.m4v")
+        should_upload_successfully("test/files/mp4/test-iso5.mp4")
         should_upload_successfully("test/files/webm/test-512x512.webm")
         should_upload_successfully("test/files/webm/test-gbrp-vp9.webm")
         # should_upload_successfully("test/files/compressed.swf")
diff --git a/test/unit/media_file_test.rb b/test/unit/media_file_test.rb
index e79e5d2e3..6bf8efa56 100644
--- a/test/unit/media_file_test.rb
+++ b/test/unit/media_file_test.rb
@@ -221,6 +221,7 @@ class MediaFileTest < ActiveSupport::TestCase
       assert_equal("yuv420p", MediaFile.open("test/files/mp4/test-300x300.mp4").pix_fmt)
       assert_equal("yuv420p", MediaFile.open("test/files/mp4/test-audio.m4v").pix_fmt)
       assert_equal("yuv420p", MediaFile.open("test/files/mp4/test-audio.mp4").pix_fmt)
+      assert_equal("yuv420p", MediaFile.open("test/files/mp4/test-300x300-iso4.mp4").pix_fmt)
       assert_equal("yuv420p", MediaFile.open("test/files/mp4/test-iso5.mp4").pix_fmt)
       assert_equal("yuv444p", MediaFile.open("test/files/mp4/test-300x300-yuv444p-h264.mp4").pix_fmt)
       assert_equal("yuvj420p", MediaFile.open("test/files/mp4/test-300x300-yuvj420p-h264.mp4").pix_fmt)
@@ -237,6 +238,8 @@ class MediaFileTest < ActiveSupport::TestCase
       assert_equal(true, MediaFile.open("test/files/mp4/test-300x300.mp4").is_supported?)
       assert_equal(true, MediaFile.open("test/files/mp4/test-300x300-vp9.mp4").is_supported?)
       assert_equal(true, MediaFile.open("test/files/mp4/test-300x300-yuvj420p-h264.mp4").is_supported?)
+      assert_equal(true, MediaFile.open("test/files/mp4/test-300x300-iso4.mp4").is_supported?)
+      assert_equal(true, MediaFile.open("test/files/mp4/test-300x300-3gp5.mp4").is_supported?)
 
       assert_equal(false, MediaFile.open("test/files/mp4/test-300x300-h265.mp4").is_supported?)
       assert_equal(false, MediaFile.open("test/files/mp4/test-300x300-av1.mp4").is_supported?)