Disregard this, setting the mime type apparently did work.
I tried submitting my Pucked game to Firefox's App store, and failed validation with:
---
Your app failed validation with 1 error.
Manifests must be served with the HTTP header "Content-Type: application/x-web-app-manifest+json". See developer.mozilla.org/en/Apps/Manifest for more information.
---
in the validation details:
---
JSON Parse Error
Error: The webapp extension could not be parsed due to a syntax error in the JSON.
Manifests must be served with the HTTP header "Content-Type: application/x-web-app-manifest+json". See developer.mozilla.org/en/Apps/Manifest for more information.
---
edit: I tried adding the mime type to my web.config file:
<staticContent>
<mimeMap fileExtension=".jps" mimeType="image/x-jps" />
<mimeMap fileExtension=".ogg" mimeType="audio/ogg" />
<mimeMap fileExtension=".m4a" mimeType="audio/mp4" />
<mimeMap fileExtension=".webapp" mimeType="application/x-web-app-manifest+json" />
</staticContent>
on the offchance it just wasn't seeing it, but that made no difference. (unless I'm doing that wrong.)