Scirra dev here. This is indeed a problem. The thing is Chrome is ultra-strict about the MIME type and rejects it if it's wrong. Firefox seems happy with it. Given HTML5 is new and nobody has their servers set up with this MIME type, and lots of people are already making HTML5 games with our tool, I'm tempted to say it's Chrome's fault for being too strict... getting everyone to reconfigure their servers is a real PITA just because Chrome is fussy.