According to this HTML5 compatibility comparator, Safari supports only partially the media capture feature: it complies with the accept
attribute but ignores the catpure
attribute.
So I am afraid you will have to wait for a better support from Safari before it works the way you want...
What's more, according to the W3C specifications:
When the capture attribute is specified, the user agent SHOULD invoke a file picker of the specific capture control type.
The keyword SHOULD means that Safari can choose not to follow the recommendation for some reason and still claim to be compliant with the standard...