IOS Upload Images To S3 With Pre Signed URLs From Back-End

I am trying to upload images from an IOS App with a Rails Back-end server to Amazon s3.

Is this the flow of the upload?

-> IOS Client

1. -> Request Back-End for Pre Signed URL

2. -> Back-End makes a request to S3 for pre-signed URL

3. -> Back-End sends pre-signed URL to IOS

4. -> IOS client uploads to S3 with pre-signed URL

Or am I missing something?