I now added a test to apport's test suite which reproduces this and potential other situations: invalid UTF-8 in description and attachment at both initial bug filing (+storeblob) and later update (launchpadlib addAttachment()).
I now added a test to apport's test suite which reproduces this and potential other situations: invalid UTF-8 in description and attachment at both initial bug filing (+storeblob) and later update (launchpadlib addAttachment()).