Advertise supported MIME types for statuses (#2090)

app/lib/html_aware_formatter.rb

@@ -1,6 +1,8 @@
 # frozen_string_literal: true
 
 class HtmlAwareFormatter
+  STATUS_MIME_TYPES = %w(text/plain text/markdown text/html).freeze
+
   attr_reader :text, :local, :options
 
   alias local? local

app/serializers/rest/instance_serializer.rb

@@ -55,6 +55,7 @@ class REST::InstanceSerializer < ActiveModel::Serializer
         max_characters: StatusLengthValidator::MAX_CHARS,
         max_media_attachments: 4,
         characters_reserved_per_url: StatusLengthValidator::URL_PLACEHOLDER_CHARS,
+        supported_mime_types: HtmlAwareFormatter::STATUS_MIME_TYPES,
       },
 
       media_attachments: {

app/serializers/rest/v1/instance_serializer.rb

@@ -79,6 +79,7 @@ class REST::V1::InstanceSerializer < ActiveModel::Serializer
         max_characters: StatusLengthValidator::MAX_CHARS,
         max_media_attachments: 4,
         characters_reserved_per_url: StatusLengthValidator::URL_PLACEHOLDER_CHARS,
+        supported_mime_types: HtmlAwareFormatter::STATUS_MIME_TYPES,
       },
 
       media_attachments: {