Since [1] the mimetype used when calling `modify_image` was obtained from the data URL contained in the `src` attribute instead of the dataset. This is wrong for background images because they do not have an `src` attribute. Thanks to [2] which repaired the value contained in `dataset.mimetype`, this value can be used as it was before [1]. [1]: https://github.com/odoo/odoo/commit/0449fe85cb0e1d639a4e1aeba26e90906f79254d [2]: https://github.com/odoo/odoo/commit/40811e72c50faf146ccdf9f26c332db9ebc69fa9 task-3446438 closes odoo/odoo#130322 X-original-commit: e3221fe5d5efa824365210f040e93654e0a30309 Signed-off-by: Romain Derie (rde) <rde@odoo.com> Signed-off-by: Benoit Socias (bso) <bso@odoo.com>