"Flat 4" style makes a litle bit trouble when you use it with the Simple Blog addon. Try to use the original Xenforo style or a other one.
Attachment upload in blogs works in certain cases only with enough php memory (256 MB or more) set on server and domain side.
I have tested it with Xenforo 1.5.4 up to 1.5.9 with every single version and my fork Simple Blogs - Attachment works like it should. with 192-256 MB php memory limit set.
But on the same domain, in a other Xenforo 1.5.9 installation with 15-20 addons and the same version from my Simple Blogs the attachment upload in the blogs sucks with less then 256MB php memory limit set. Why? Good question...
Beta - what means that?
That means that the addon is not 100% ready for use, last errors can found and make trouble. So use it of your own risk - take a backup bevor you install this and test it in a copy of your live system bevor you run it in a forum with members.
Support?
You get what you pay for. Ok, thats not realy funny but be aware that I code only in my free time after family and work, so it can be, that I dont can answer or make a fix in the next hour for your special issue. But I do my best and I ever answer on requests.