uploads: fix it being possible to upload .mkv files as .webm.

Fix it being possible to upload arbitrary .mkv files and have them
be treated as .webm. This was possible because WebM uses the Matroska
container format, and we only checked for the Matroska header, not that
the file was actually a WebM.

There were only 6 such files in production:

* https://danbooru.donmai.us/posts?tags=exif:Matroska:DocType=matroska
* https://danbooru.donmai.us/posts/5522036
* https://danbooru.donmai.us/posts/4743498
* https://danbooru.donmai.us/posts/3925427
* https://danbooru.donmai.us/posts/3147897
* https://danbooru.donmai.us/posts/2965862
* https://danbooru.donmai.us/posts/2430436

These videos are playable in Chrome, but not in Firefox, since Firefox
doesn't support .mkv files (it supports some, depending on which codecs
are used, but not .mkv files in general).

test/components/post_preview_component_test.rb

@@ -20,7 +20,7 @@ class PostPreviewComponentTest < ViewComponent::TestCase
 
     context "for a video post" do
       should "render" do
-        @post = create(:post_with_file, filename: "test-512x512.webm").reload
+        @post = create(:post_with_file, filename: "webm/test-512x512.webm").reload
         node = render_preview(@post, current_user: User.anonymous)
 
         assert_equal(post_path(@post), node.css("article a").attr("href").value)

test/functional/uploads_controller_test.rb

@@ -224,6 +224,13 @@ class UploadsControllerTest < ActionDispatch::IntegrationTest
         end
       end
 
+      context "for an unsupported video file" do
+        should "fail for a .mkv file" do
+          create_upload!("test/files/webm/test-512x512.mkv", user: @user)
+          assert_match("File type is not supported", Upload.last.error)
+        end
+      end
+
       context "for a video longer than the video length limit" do
         should "fail for a regular user" do
           create_upload!("https://cdn.donmai.us/original/63/cb/63cb09f2526ef3ac14f11c011516ad9b.webm", user: @user)
@@ -293,7 +300,7 @@ class UploadsControllerTest < ActionDispatch::IntegrationTest
         should_upload_successfully("test/files/test-static-32x32.gif")
         should_upload_successfully("test/files/test-animated-86x52.gif")
         should_upload_successfully("test/files/test-300x300.mp4")
-        should_upload_successfully("test/files/test-512x512.webm")
+        should_upload_successfully("test/files/webm/test-512x512.webm")
         should_upload_successfully("test/files/test-audio.m4v")
         # should_upload_successfully("test/files/compressed.swf")
 

test/unit/media_file_test.rb

@@ -27,7 +27,7 @@ class MediaFileTest < ActiveSupport::TestCase
 
     should "determine the correct dimensions for a webm file" do
       skip unless MediaFile.videos_enabled?
-      assert_equal([512, 512], MediaFile.open("test/files/test-512x512.webm").dimensions)
+      assert_equal([512, 512], MediaFile.open("test/files/webm/test-512x512.webm").dimensions)
     end
 
     should "determine the correct dimensions for a mp4 file" do
@@ -58,7 +58,7 @@ class MediaFileTest < ActiveSupport::TestCase
     should "work for a video if called twice" do
       skip unless MediaFile.videos_enabled?
 
-      mf = MediaFile.open("test/files/test-512x512.webm")
+      mf = MediaFile.open("test/files/webm/test-512x512.webm")
       assert_equal([512, 512], mf.dimensions)
       assert_equal([512, 512], mf.dimensions)
 
@@ -96,7 +96,7 @@ class MediaFileTest < ActiveSupport::TestCase
     end
 
     should "determine the correct extension for a webm file" do
-      assert_equal(:webm, MediaFile.open("test/files/test-512x512.webm").file_ext)
+      assert_equal(:webm, MediaFile.open("test/files/webm/test-512x512.webm").file_ext)
     end
 
     should "determine the correct extension for a mp4 file" do
@@ -150,7 +150,7 @@ class MediaFileTest < ActiveSupport::TestCase
 
     should "generate a preview image for a video" do
       skip unless MediaFile.videos_enabled?
-      assert_equal([150, 150], MediaFile.open("test/files/test-512x512.webm").preview(150, 150).dimensions)
+      assert_equal([150, 150], MediaFile.open("test/files/webm/test-512x512.webm").preview(150, 150).dimensions)
       assert_equal([150, 150], MediaFile.open("test/files/test-300x300.mp4").preview(150, 150).dimensions)
     end
 
@@ -204,11 +204,17 @@ class MediaFileTest < ActiveSupport::TestCase
 
   context "for a webm file" do
     should "determine the duration of the video" do
-      file = MediaFile.open("test/files/test-512x512.webm")
+      file = MediaFile.open("test/files/webm/test-512x512.webm")
       assert_equal(0.48, file.duration)
       assert_equal(10/0.48, file.frame_rate)
       assert_equal(10, file.frame_count)
     end
+
+    should "not detect .mkv files as .webm" do
+      file = MediaFile.open("test/files/webm/test-512x512.mkv")
+
+      assert_equal(false, file.is_supported?)
+    end
   end
 
   context "a compressed SWF file" do

test/unit/post_query_builder_test.rb

@@ -775,7 +775,7 @@ class PostQueryBuilderTest < ActiveSupport::TestCase
     end
 
     should "return posts for the duration:<x> metatag" do
-      post = create(:post, media_asset: create(:media_asset, file: "test/files/test-512x512.webm"))
+      post = create(:post, media_asset: create(:media_asset, file: "test/files/webm/test-512x512.webm"))
 
       assert_tag_match([post], "duration:0.48")
       assert_tag_match([post], "duration:>0.4")