BUG: Requesting image that doesn't exist returns `Content-Type: json`

[Owain Cuvelier]

A bug in Tachyon seems to be; if you make a request to an image via Tachyon, and the image doesn't exist, it'll return a Content-Type header with value JSON, which isn't desirable.

[Joe Hoyle]

Need to check if this is still the case with #162