To attach files you have to chose "Use advanced editor" at the upper right, then you are given the opportunity to attach files at the end of the post. Alternatively you can post, "Add Reply" and then immediatly chose edit - thos brings you in advanced editor, too.
The braces you see in the evaluation of M are just an indicator of nested arrays. {2,2} means that this is a 2x2 matrix, but thats explained in text next to the expression anyway. You can try to double click the result (or chose Format Result from the menu) and then in tab "Display Options" check "Expand nested arrays".