Add customizable thumbnails for audio and video attachments (#14145)

- Change audio files to not be stripped of metadata
- Automatically extract cover art from audio if it exists
- Add `thumbnail` parameter to `POST /api/v1/media`, `POST /api/v2/media` and `PUT /api/v1/media/:id`
- Add `icon` to represent it in attachments in ActivityPub
- Fix `preview_url` containing URL of missing missing image when there is no thumbnail instead of null
- Fix duration of audio not being displayed on public pages until the file is loaded

lib/paperclip/image_extractor.rb

@@ -0,0 +1,49 @@
+# frozen_string_literal: true
+
+require 'mime/types/columnar'
+
+module Paperclip
+  class ImageExtractor < Paperclip::Processor
+    IMAGE_EXTRACTION_OPTIONS = {
+      convert_options: {
+        output: {
+          'loglevel' => 'fatal',
+          vf: 'scale=\'min(400\, iw):min(400\, ih)\':force_original_aspect_ratio=decrease',
+        }.freeze,
+      }.freeze,
+      format: 'png',
+      time: -1,
+      file_geometry_parser: FastGeometryParser,
+    }.freeze
+
+    def make
+      return @file unless options[:style] == :original
+
+      image = begin
+        begin
+          Paperclip::Transcoder.make(file, IMAGE_EXTRACTION_OPTIONS.dup, attachment)
+        rescue Paperclip::Error, ::Av::CommandError
+          nil
+        end
+      end
+
+      unless image.nil?
+        begin
+          attachment.instance.thumbnail = image if image.size.positive?
+        ensure
+          # Paperclip does not automatically delete the source file of
+          # a new attachment while working on copies of it, so we need
+          # to make sure it's cleaned up
+
+          begin
+            FileUtils.rm(image)
+          rescue Errno::ENOENT
+            nil
+          end
+        end
+      end
+
+      @file
+    end
+  end
+end