Change video preview format from PNG to JPG

activestorage/lib/active_storage/previewer/video_previewer.rb

@@ -9,15 +9,14 @@ module ActiveStorage
     def preview
       download_blob_to_tempfile do |input|
         draw_relevant_frame_from input do |output|
-          yield io: output, filename: "#{blob.filename.base}.png", content_type: "image/png"
+          yield io: output, filename: "#{blob.filename.base}.jpg", content_type: "image/jpeg"
         end
       end
     end
 
     private
       def draw_relevant_frame_from(file, &block)
-        draw ffmpeg_path, "-i", file.path, "-y", "-vcodec", "png",
-          "-vf", "thumbnail", "-vframes", "1", "-f", "image2", "-", &block
+        draw ffmpeg_path, "-i", file.path, "-y", "-vframes", "1", "-f", "image2", "-", &block
       end
 
       def ffmpeg_path

activestorage/test/models/preview_test.rb

@@ -22,8 +22,8 @@ class ActiveStorage::PreviewTest < ActiveSupport::TestCase
     preview = blob.preview(resize: "640x280").processed
 
     assert_predicate preview.image, :attached?
-    assert_equal "video.png", preview.image.filename.to_s
-    assert_equal "image/png", preview.image.content_type
+    assert_equal "video.jpg", preview.image.filename.to_s
+    assert_equal "image/jpeg", preview.image.content_type
 
     image = read_image(preview.image)
     assert_equal 640, image.width

activestorage/test/previewer/video_previewer_test.rb

@@ -12,12 +12,13 @@ class ActiveStorage::Previewer::VideoPreviewerTest < ActiveSupport::TestCase
 
   test "previewing an MP4 video" do
     ActiveStorage::Previewer::VideoPreviewer.new(@blob).preview do |attachable|
-      assert_equal "image/png", attachable[:content_type]
-      assert_equal "video.png", attachable[:filename]
+      assert_equal "image/jpeg", attachable[:content_type]
+      assert_equal "video.jpg", attachable[:filename]
 
       image = MiniMagick::Image.read(attachable[:io])
       assert_equal 640, image.width
       assert_equal 480, image.height
+      assert_equal "image/jpeg", image.mime_type
     end
   end
 end