aws-sam local tmp directory

I'm working on some functions to generate documents and email them off, that all works but it's hard to debug waiting for the email.

I write the file to the lambda tmp folder, and I can see it there by listing the directory in my code, but I can't find a way to access it locally.

I've seen the discussion about setting and environment variable "TMPDIR", but that seems not to work, either inline of using the --env-vars env.json

{
  "Parameters": {
        "TMPDIR": "/tmp"
  }
}

I'm running sam on Linux

Any pointers would be great.