Packer HCL2 config file support - configuration

Question

In https://packer.io/guides/hcl/from-json-v1/, it says
Note: Starting from version 1.5.0 Packer can read HCL2 files.
And my packer is packer_1.5.5_linux_amd64.zip which is suppose to be able to read HCL2 files. However, when I tried it, I got
$ packer build -only=docker hcl-example
Failed to parse template: Error parsing JSON: invalid character '#' looking for beginning of value
At line 1, column 1 (offset 1):
1: #
^
==> Builds finished but no artifacts were created.
$ packer build -h
Usage: packer build [options] TEMPLATE
Will execute multiple builds in parallel as defined in the template.
The various artifacts created by the template will be outputted.
Options:
-color=false Disable color output. (Default: color)
-debug Debug mode enabled for builds.
-except=foo,bar,baz Run all builds and post-procesors other than these.
-only=foo,bar,baz Build only the specified builds.
-force Force a build to continue if artifacts exist, deletes existing artifacts.
-machine-readable Produce machine-readable output.
-on-error=[cleanup|abort|ask] If the build fails do: clean up (default), abort, or ask.
-parallel=false Disable parallelization. (Default: true)
-parallel-builds=1 Number of builds to run in parallel. 0 means no limit (Default: 0)
-timestamp-ui Enable prefixing of each ui output with an RFC3339 timestamp.
-var 'key=value' Variable for templates, can be used multiple times.
-var-file=path JSON file containing user variables. [ Note that even in HCL mode this expects file to contain JSON, a fix is comming soon ]
and I don't see any switches from above to switch to HCL2 mode.
What I'm missing here?
$ packer version
Packer v1.5.5
$ cat hcl-example
# the source block is what was defined in the builders section and represents a
# reusable way to start a machine. You build your images from that source.source
"amazon-ebs" "example" {
ami_name = "packer-test"
region = "us-east-1"
instance_type = "t2.micro"
}
[UPDATE:]
To address Matt's comment/concern, I've changed the content of hcl-example to the whole list in https://packer.io/guides/hcl/from-json-v1/, and
mv hcl-example hcl-example.hcl
$ packer validate hcl-example.hcl
Failed to parse template: Error parsing JSON: invalid character '#' looking for beginning of value
At line 1, column 1 (offset 1):
1: #
^

Answer 1

Named it with .pkr.hcl extension solved the problem.