This one's a bit obscure. I've got a post which is a share of an album from Google Play Music. When you view the post, it's a complicated attachment containing "Listen", "Buy" links and so on. But when you grab the post via the SDK, all you get is a title, an image of the album cover and a link. Basically the SDK attachment doesn't really reflect the post very well. The attachment type is "objectType": "audio".

Now I guess you could say the same thing about Youtube. But this means that somebody using the API needs to be aware of and build a representation of every possible attachment.

Post: https://plus.google.com/106416716945076707395/posts/YhLavWoi9Qy
Post ID: z12vhzkjrue3etjxx04ccnti2wb4ejnxysk0k

Returned:
 "attachments": [
     {
      "objectType": "audio",
      "displayName": "Rye Lane Volume One",
      "url": "https://play.google.com/music/playpreview?preview=AE9vGKq9bZV1vP2JUhloXCtqm09d63mts1p_kwNro89uJH7V1vXzSLAzZ9GsdU6eOHKF_-IXrP8T5TOdU3l9Q2SP5uRtUzWOxyaiYzFii6fwAVblzgQnxZg%3D",
      "image": {
       "url": "https://lh6.googleusercontent.com/HTuBpdf2SAvpcUxl5x_oN6mWr0Fld4MpprrMsXrWR0xuMbc8pD6gwf9z1nW9QxC9iNvmibmd",
       "type": "image/jpeg"
      }
     }
    ]