Serving a .dmg via readfile?

Hey all - I'm having no luck serving a .dmg from my online store. I stripped down the code to just the following to debug, but no matter what I get a zero-byte file served: header('Content-Type: application/x-apple-diskimage'); // also tried octet-stream header('Content-Disposition: attachment; filename="My Cool Image.dmg"'); $size = filesize('/var/www/mypath/My Cool Image.dmg'); header('Content-Length: '.$size); readfile('/var/www/mypath/My Cool Image.dmg'); This same code works for a number of other file types that I serve: bin, zip, pdf. Any suggestions? Professor Google is not my friend.