Add `configuration` attribute to `GET /api/v1/instance` (#16485)

List various values like file size limits and supported mime types

app/serializers/rest/instance_serializer.rb

@@ -5,7 +5,8 @@ class REST::InstanceSerializer < ActiveModel::Serializer
 
   attributes :uri, :title, :short_description, :description, :email,
              :version, :urls, :stats, :thumbnail,
-             :languages, :registrations, :approval_required, :invites_enabled
+             :languages, :registrations, :approval_required, :invites_enabled,
+             :configuration
 
   has_one :contact_account, serializer: REST::AccountSerializer
 
@@ -53,6 +54,32 @@ class REST::InstanceSerializer < ActiveModel::Serializer
     { streaming_api: Rails.configuration.x.streaming_api_base_url }
   end
 
+  def configuration
+    {
+      statuses: {
+        max_characters: StatusLengthValidator::MAX_CHARS,
+        max_media_attachments: 4,
+        characters_reserved_per_url: StatusLengthValidator::URL_PLACEHOLDER_CHARS,
+      },
+
+      media_attachments: {
+        supported_mime_types: MediaAttachment::IMAGE_MIME_TYPES + MediaAttachment::VIDEO_MIME_TYPES + MediaAttachment::AUDIO_MIME_TYPES,
+        image_size_limit: MediaAttachment::IMAGE_LIMIT,
+        image_matrix_limit: Attachmentable::MAX_MATRIX_LIMIT,
+        video_size_limit: MediaAttachment::VIDEO_LIMIT,
+        video_frame_rate_limit: MediaAttachment::MAX_VIDEO_FRAME_RATE,
+        video_matrix_limit: MediaAttachment::MAX_VIDEO_MATRIX_LIMIT,
+      },
+
+      polls: {
+        max_options: PollValidator::MAX_OPTIONS,
+        max_characters_per_option: PollValidator::MAX_OPTION_CHARS,
+        min_expiration: PollValidator::MIN_EXPIRATION,
+        max_expiration: PollValidator::MAX_EXPIRATION,
+      },
+    }
+  end
+
   def languages
     [I18n.default_locale]
   end

app/validators/status_length_validator.rb

@@ -2,7 +2,8 @@
 
 class StatusLengthValidator < ActiveModel::Validator
   MAX_CHARS = 500
-  URL_PLACEHOLDER = "\1#{'x' * 23}"
+  URL_PLACEHOLDER_CHARS = 23
+  URL_PLACEHOLDER = "\1#{'x' * URL_PLACEHOLDER_CHARS}"
 
   def validate(status)
     return unless status.local? && !status.reblog?