Does this help? Notice the density - ie fat.

Answer: Bochdalek hernia

Probably wise to CT this if there is no p.rior or without any history. Ideally a mass should be a little more dense on the plain film, but you never know what is popping up there along with fat - ie colon, kidney, spleen, etc

reference: http://radiopaedia.org/articles/bochdalek_hernia