The "application/x-web-app-manifest+json" content-type is used for webapp manifests.
1) Open web console
2) Enable "Log Request and Response bodies"
3) Load load manifest URL (see URL field)
4) Click on HTTP request for .webapp
Unable to display responses of type ""
Show the response body like we do for JSON.
Thank you for the bug report!
I have added the MIME type you suggest to the list of known types for the Web Console, in patch "part 4" from bug 673148.
Marking this bug as fixed since the patch for bug 673148 included the fix for it.