A simple trick to add additional bytes prepending Flask's stream_with_context

  from flask_admin.contrib.sqla import ModelView
  from itertools import chain

  class MyAdminView(ModelView):
      can_export = True
      column_export_list = ['field1', 'field2']

      def _export_csv(self, return_url):
          r = super(MyAdminView, self)._export_csv(return_url)
          r.response = chain((b'\xef\xbb\xbf',), r.response)
          return r

I am still using Python2 and Flask-admin shit. It's a shame but anyway.