app/models/concerns/attachmentable.rb

@@ -24,7 +24,7 @@ module Attachmentable
     def self.has_attached_file(name, options = {}) # rubocop:disable Naming/PredicateName
       super(name, options)
 
-      send(:"before_#{name}_validate") do
+      send(:"before_#{name}_validate", prepend: true) do
         attachment = send(name)
         check_image_dimension(attachment)
         set_file_content_type(attachment)

spec/requests/api/v2/media_spec.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe 'Media API', paperclip_processing: true do
+  let(:user)    { Fabricate(:user) }
+  let(:token)   { Fabricate(:accessible_access_token, resource_owner_id: user.id, scopes: scopes) }
+  let(:scopes)  { 'write' }
+  let(:headers) { { 'Authorization' => "Bearer #{token.token}" } }
+
+  describe 'POST /api/v2/media' do
+    it 'returns http success' do
+      post '/api/v2/media', headers: headers, params: { file: fixture_file_upload('attachment-jpg.123456_abcd', 'image/jpeg') }
+      expect(File.exist?(user.account.media_attachments.first.file.path(:small))).to be true
+      expect(response).to have_http_status(200)
+    end
+  end
+end