“Binary Property” does not have to be an expression. It is just a string. This is however not the problem here, just to be complete.
The problem you have is caused by you receiving multiple messages (in this case 10) and only one having an attachment and so binary data. So it will only work for 1 of the 10 and for 9 of 10 it will display the error you can see (as there is nothing the Dropbox node could upload).
So what you would have to do is to filter the items(messages) with an IF-Node. So that the Dropbox node will only be executed for the items that contain binary data.