Blob.AttachOnDocument doesn't handle correctly list of blobs

While working on JAVACLIENT-179 we've notice that Blob.AttachOnDocument (for exmaple, can happen for other operations) doesn't handle correctly a list of blobs in a specific context.

The first thing we observed is that the operation is saving the document after adding each blobs, which could be a performance issue and which triggers too many documentUpdated events.

The second is that only the first blob of the list is saved when the full document is given as context param, like for example:

                {
                  "params": {
                    "document": {
                      "entity-type": "document",
                      "uid": SOME_UID",
                      "properties": {
                        "files:files": []
                      }
                    },
                    "xpath": "files:files"
                  }
                }

We would like to fix the second point and optimize the first one.