Ah that was tackled in that genetic syntax thread as well Sam. Basically it can be thought of as “syntactic sugar” and auto converted to the full {} form internally.
http://talk.commonmark.org/t/consistent-attribute-syntax/272/22?u=mofosyne
Here is a copy of the relevant section:
Embedded Media
!mediaType[description](url){#myId .myClass key=val key2="val 2"}
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assumed to be image if mediaType left blank
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syntactic sugar ( content of
()handled by mediaType handler/extension):![](file.mp4 "video title" 80x10 ) is equivalent to typing: !video[](file.mp4){title="video title" width=80 height=10}