image upload field: fixed for change form if file was already uploaded
image upload field: fixed for change form if file was already uploaded

jmagnusson/flask-admin,flask-admin/flask-admin,rochacbruno/flask-admin,flask-admin/flask-admin,flask-admin/flask-admin,flask-admin/flask-admin,jmagnusson/flask-admin,jmagnusson/flask-admin,rochacbruno/flask-admin,jmagnusson/flask-admin,rochacbruno/flask-admin,rochacbruno/flask-admin