amazon-import not supported?

[telepath]

I have a build that creates an AMI using the amazon-import post-processor, which does not support encryption.
For this, I'd like to use the ami-copy post-processor to encrypt the imported ami, however the module does not accept that artifact as input:

Build 'file' errored: 1 error(s) occurred:                                                                                                                     
                                                                              
* Post-processor failed: Unexpected artifact type: packer.file
Can only export from Amazon builders       

I'm using the file builder to import an existing ova file, like this:

{
  "builders":
  [{
    "type": "file",
    "source": "files/appliance.ova",
    "target": "output-file/appliance.ova"
  }],
  "post-processors":
  [{
    "type": "amazon-import",
    "s3_bucket_name": "vmimportbucket",
    "ami_name": "appliance-import",
    "license_type": "BYOL"
  },
  {
    "type": "ami-copy",
    "ami_users": "{{ account_id }}",
    "kms_key_id": "{{ kms_key_id }}",
    "encrypt_boot": true,
    "ensure_available": true
  }]
}