The idea is an user can upload a *.bpmn file to a wike.
Well, for the wiki part of the job, it seems that what you want is just a modified version of the Graphviz extension. This extension converts DOT notation to PNG images (using a supplied converter), manages a cache of the results, and supplies the image when the graph is requested. So, if you have a suitable BPMN-to-image converter, and moderate PHP skills, adapting the Graphviz extension should be fairly easy.
I guess the problem is finding or writing a BPMN-to-image converter. Like ImageMagick, such a converter wouldn't be specific to MediaWiki; so this might not be the best place to ask for that.
Another tack might be to write a BPMN-to-DOT converter, then just use Graphviz.
Hope this helps (but probably not much),
Ian Smith