class ApplicationController < ActionController::API
  include ActionController::StrongParameters

  def authenticate_user_from_token!
    user_email = params[:user_email].presence
    user       = user_email && User.find_by_email(user_email)

    # Notice how we use Devise.secure_compare to compare the token
    # in the database with the token given in the params, mitigating
    # timing attacks.
    if user && Devise.secure_compare(user.authentication_token, params[:user_token])
      sign_in user, store: false
    else
      render :json => { :errors => ["Unknown auth token"] }, :success => false, :status => :unauthorized
    end
  end
end

class ApplicationController < ActionController::API
  include ActionController::StrongParameters
  include ActionController::MimeResponds

  def authenticate_user_from_token!
    user_email = params[:user_email].presence
    user       = user_email && User.find_by_email(user_email)

    # Notice how we use Devise.secure_compare to compare the token
    # in the database with the token given in the params, mitigating
    # timing attacks.
    if user && Devise.secure_compare(user.authentication_token, params[:user_token])
      sign_in user, store: false
    else
      render :json => { :errors => ["Unknown auth token"] }, :success => false, :status => :unauthorized
    end
  end
end

Add MimeResponds module for Devise
Add MimeResponds module for Devise
