CLI env parameter value can't be read from file

According to documentation there is an option to read parameter value from file with @.
It doesn't work for env parameter.

From string (OK):

# st2 run core.local env='a=1' cmd='echo "ponies ${a} ${b}"'
# st2 run core.local env='a=1,b=2' cmd='echo "ponies ${a} ${b}"'
# st2 run core.local env='{"a": "1", "b": "2"}' cmd='echo "ponies ${a} ${b}"'
.
id: 56327b893ee8aa60bee3c2a5
status: succeeded
parameters:
  cmd: echo "ponies ${a} ${b}"
  env:
    a: '1'
    b: '2'
result:
  failed: false
  return_code: 0
  stderr: ''
  stdout: ponies 1 2
  succeeded: true

Same from file (ERROR):

# echo '{"a": "1", "b": "2"}' > env.json

# st2 run core.local @env=env.json cmd='echo "ponies ${a} ${b}"'
ERROR: 400 Client Error: Bad Request
MESSAGE: '{"a": "1", "b": "2"}\n' is not of type 'object' for url: https://172.30.0.119:9101/executions

and

# echo 'a=1' > var.conf

# st2 run core.local @env=var.conf cmd='echo "ponies ${a} ${b}"'
ERROR: 400 Client Error: Bad Request
MESSAGE: 'a=1\n' is not of type 'object' for url: https://172.30.0.119:9101/executions

Start by reproducing the documented @file behavior with the st2 run CLI and compare it with inline env values. Trace how file-loaded parameters are parsed before the execution request; done means JSON and key-value environment files are accepted as an object and the documented command succeeds.