(#2773) - attachments work after Chrome 37-38 upgrade

No tests added, because I don't know if we can write a test that upgrades a Saucelabs browser from Chrome 37 to 38. ;)

But the fix is pretty simple. We only care about the `blobSupport` variable when we write blobs. When we read blobs, we simply detect whether what we got back was a string or an object (or null/undefined).

Tested manually in Firefox and Chrome 35, 36, and 38. Would like a green on IE10 as well before we merge this.