To say the least, it is a strange picture. It is much too neat for what we have in mind as what "Confederates" would look like. And I think that is the problem. They don't look like Confederates but they don't look like Unionist either. The light colored uniforms, the neatness of the Uniforms, the kepi caps all seem to be too regulation for what we think of as Confederate soldiers.
However, we must keep an open mind here. Where, when and why was the picture taken? It is obviously a "Staged" setting for the photographers benefit. If it is a Confederate field hospital, it would have been staged in the Richmond Va. area where such "regulation" uniforms and equipment could have been available. And taken to impress someone, maybe the English? Possibly during the Seven Days battle where massive amounts of Federal equipment was secured and Photographers were wanting to document everything.