title: Amazon S3 policy allowing only upload not overwrite
body: <p>I'm developing a mobile application which will let anyone upload a file to an S3 bucket. I think I'll use the Anonymous Token Vending Machine that is provided by Amazon.</p>
  
  <p>However, I can't see how to write a <code>TokenVendingMachinePolicy.json</code> file that will only allow uploading new files, not overwriting (effectively deleting). I thought allowing just <code>s3:PutObject</code> would be fine, but it appears the <code>s3:PutObject</code> permission allows both. This seems a bit strange, since there's a separate <code>s3:DeleteObject</code> permission, which is useless, since you can just overwrite with 0-length object anyway.</p>
  
  <p>Is there any way to only allow uploading new files? How is this problem usually solved?</p>