Turn off attachment previews on large files

ref: https://fizzy.37signals.com/5986089/collections/7/cards/1302

test/lib/rails_ext/active_storage_blob_previewable_test.rb

@@ -0,0 +1,13 @@
+require "test_helper"
+
+class ActiveStorageBlobPreviewableTest < ActiveSupport::TestCase
+  test "small files are previewable" do
+    blob = ActiveStorage::Blob.new(filename: "test.mp4", content_type: "video/mp4", byte_size: 1024 * 1024, key: "test", checksum: "abc")
+    assert blob.previewable?
+  end
+
+  test "large files are not previewable" do
+    blob = ActiveStorage::Blob.new(filename: "test.mp4", content_type: "video/mp4", byte_size: ActiveStorageBlobPreviewable::MAX_PREVIEWABLE_SIZE + 1, key: "test", checksum: "abc")
+    assert_not blob.previewable?
+  end
+end