It's a little bit different because the Settings.mmsSizeLimitation means the limitation for mms message, but the defaultLimit here means the limitation for single image attachment. Ideally we don't have to consider the Settings.mmsSizeLimitation here because it should be controlled by caller. The 300k image limitation is based on the original mms user story(https://bugzilla.mozilla.org/show_bug.cgi?id=840038) and the it's possible that mmsSizeLimitation is much bigger than 300k(maybe 600k or more) and that will make sure user could append more than one image.