Illustrator .ai files are previewable as PDFs

This happened to work with Marcel 1.0.2 and earlier since magic byte
sniffing sees that Illustrator files are PDFs internally, causing these
files to be treated as `application/pdf` despite having a declared
content type of `application/illustrator` and an `.ai` file extension.

Marcel 1.0.3 corrected this to the more specific `application/illustrator`
subtype of `application/pdf`, but the MuPDF previewer only accepts the
parent `application/pdf` type.

Changing it to accept PDF and any child types allows the previewer to
explicitly work with Illustrator files again, which was only a happy
accident previously.

activestorage/lib/active_storage/previewer/mupdf_previewer.rb

@@ -4,7 +4,11 @@ module ActiveStorage
   class Previewer::MuPDFPreviewer < Previewer
     class << self
       def accept?(blob)
-        blob.content_type == "application/pdf" && mutool_exists?
+        pdf?(blob.content_type) && mutool_exists?
+      end
+
+      def pdf?(content_type)
+        Marcel::Magic.child? content_type, "application/pdf"
       end
 
       def mutool_path

activestorage/test/previewer/mupdf_previewer_test.rb

@@ -32,6 +32,19 @@ class ActiveStorage::Previewer::MuPDFPreviewerTest < ActiveSupport::TestCase
     end
   end
 
+  test "previewing an Illustrator document that's a PDF subtype" do
+    blob = create_file_blob(fixture: "report.pdf", filename: "file.ai", content_type: "application/illustrator")
+
+    ActiveStorage::Previewer::MuPDFPreviewer.new(blob).preview do |attachable|
+      assert_equal "image/png", attachable[:content_type]
+      assert_equal "file.png", attachable[:filename]
+
+      image = MiniMagick::Image.read(attachable[:io])
+      assert_equal 612, image.width
+      assert_equal 792, image.height
+    end
+  end
+
   test "previewing a PDF that can't be previewed" do
     blob = create_file_blob(filename: "video.mp4", content_type: "application/pdf")