Fix remote media descriptions being cut off at 420 chars

Fixes mastodon#12258

app/models/media_attachment.rb

@@ -26,6 +26,8 @@ class MediaAttachment < ApplicationRecord
 
   enum type: [:image, :gifv, :video, :unknown, :audio]
 
+  MAX_DESCRIPTION_LENGTH = 1_500
+
   IMAGE_FILE_EXTENSIONS = %w(.jpg .jpeg .png .gif).freeze
   VIDEO_FILE_EXTENSIONS = %w(.webm .mp4 .m4v .mov).freeze
   AUDIO_FILE_EXTENSIONS = %w(.ogg .oga .mp3 .wav .flac .opus .aac .m4a .3gp .wma).freeze
@@ -139,7 +141,7 @@ class MediaAttachment < ApplicationRecord
   include Attachmentable
 
   validates :account, presence: true
-  validates :description, length: { maximum: 1_500 }, if: :local?
+  validates :description, length: { maximum: MAX_DESCRIPTION_LENGTH }, if: :local?
 
   scope :attached,   -> { where.not(status_id: nil).or(where.not(scheduled_status_id: nil)) }
   scope :unattached, -> { where(status_id: nil, scheduled_status_id: nil) }
@@ -243,7 +245,7 @@ def set_shortcode
   end
 
   def prepare_description
-    self.description = description.strip[0...420] unless description.nil?
+    self.description = description.strip[0...MAX_DESCRIPTION_LENGTH] unless description.nil?
   end
 
   def set_type_and_extension

spec/lib/activitypub/activity/create_spec.rb

@@ -261,6 +261,32 @@
         end
       end
 
+
+      context 'with media attachments with long description' do
+        let(:object_json) do
+          {
+            id: [ActivityPub::TagManager.instance.uri_for(sender), '#bar'].join,
+            type: 'Note',
+            content: 'Lorem ipsum',
+            attachment: [
+              {
+                type: 'Document',
+                mediaType: 'image/png',
+                url: 'http://example.com/attachment.png',
+                name: '*' * 1500,
+              },
+            ],
+          }
+        end
+
+        it 'creates status' do
+          status = sender.statuses.first
+
+          expect(status).to_not be_nil
+          expect(status.media_attachments.map(&:description)).to include('*' * 1500)
+        end
+      end
+
       context 'with media attachments with focal points' do
         let(:object_json) do
           {