Add API to upload media attachments

app/models/media_attachment.rb

@@ -0,0 +1,13 @@
+class MediaAttachment < ApplicationRecord
+  belongs_to :account, inverse_of: :media_attachments
+  belongs_to :status,  inverse_of: :media_attachments
+
+  has_attached_file :file
+  validates_attachment_content_type :file, content_type: /\Aimage\/.*\z/
+
+  validates :account, presence: true
+
+  def local?
+    self.remote_url.blank?
+  end
+end