Here’s your answer:
[alice@localhost ~]$ curl -I https://ps.w.org/jetpack/assets/screenshot-5.png
HTTP/1.1 200 OK
Accept-Ranges: bytes
Cache-Control: max-age=31536000
Content-Type: image/png
Date: Wed, 10 Dec 2014 13:03:24 GMT
Etag: "640145//jetpack/assets/screenshot-5.png"
Expires: Thu, 10 Dec 2015 13:03:24 GMT
Last-Modified: Sun, 16 Dec 2012 19:59:35 GMT
Server: Apache
Content-Length: 33269
[alice@localhost ~]$ curl -I https://ps.w.org/jetpack/assets/screenshot-6.png
HTTP/1.1 200 OK
Accept-Ranges: bytes
Content-Type: application/octet-stream
Date: Wed, 10 Dec 2014 13:03:43 GMT
Etag: "640145//jetpack/assets/screenshot-6.png"
Last-Modified: Sun, 16 Dec 2012 19:59:35 GMT
Server: Apache
Content-Length: 361672
The second is being served with the mime type application/octet-stream.
The server is apache, which usually does mime type based upon extension if served directly by apache – so something else within the wordpress install is actually serving the image and is screwing up the mime type on some occassions.
There’s probably a mod_rewrite directive that sends serving of the files to a script and the script is mis-identifying the file type in the second instance.