[Bug] Cannot run "az devops invoke" with --in-fille different than json

**Describe the bug**
No way to upload secure files using az devops invoke command. --in-fille expects only json, where secure file in the library can be different.

az devops invoke --api-version "5.0" --area distributedtask --resource securefiles --organization "https://dev.azure.com/myorg" --http-method POST --media-type "application/octet-stream" --route-parameters project=myproject --in-file "dev.env"

**To Reproduce**
Azure Cli Version: 2.6.0
Azure-Devops extension version: 0.18.0

Steps to reproduce the behavior:
az devops invoke --api-version "5.0" --area distributedtask --resource securefiles --organization "https://dev.azure.com/myorg" --http-method POST --media-type "application/octet-stream" --route-parameters project=myproject --in-file "dev.env"

**Expected behavior**
Secure file is uploaded.
If --media-type is different than application/json than --in-file can be different type.

**Debug logs**
cli.azure.cli.core.util : Expecting value: line 1 column 1 (char 0)
Traceback (most recent call last):
File "/opt/az/lib/python3.6/site-packages/knack/cli.py", line 215, in invoke
cmd_result = self.invocation.execute(args)
File "/opt/az/lib/python3.6/site-packages/azure/cli/core/commands/__init__.py", line 654, in execute
raise ex
File "/opt/az/lib/python3.6/site-packages/azure/cli/core/commands/__init__.py", line 718, in _run_jobs_serially
results.append(self._run_job(expanded_arg, cmd_copy))
File "/opt/az/lib/python3.6/site-packages/azure/cli/core/commands/__init__.py", line 709, in _run_job
cmd_copy.exception_handler(ex)
File "/home/daporo/.azure/cliextensions/azure-devops/azext_devops/dev/common/exception_handler.py", line 31, in azure_devops_exception_handler
reraise(*sys.exc_info())
File "/opt/az/lib/python3.6/site-packages/six.py", line 703, in reraise
raise value
File "/opt/az/lib/python3.6/site-packages/azure/cli/core/commands/__init__.py", line 688, in _run_job
result = cmd_copy(params)
File "/opt/az/lib/python3.6/site-packages/azure/cli/core/commands/__init__.py", line 325, in __call__
return self.handler(*args, **kwargs)
File "/opt/az/lib/python3.6/site-packages/azure/cli/core/__init__.py", line 545, in default_command_handler
return op(**command_args)
File "/home/daporo/.azure/cliextensions/azure-devops/azext_devops/dev/team/invoke.py", line 44, in invoke
request_body = json.loads(in_file_content)
File "/opt/az/lib/python3.6/json/__init__.py", line 354, in loads
return _default_decoder.decode(s)
File "/opt/az/lib/python3.6/json/decoder.py", line 339, in decode
obj, end = self.raw_decode(s, idx=_w(s, 0).end())
File "/opt/az/lib/python3.6/json/decoder.py", line 357, in raw_decode
raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)
Expecting value: line 1 column 1 (char 0)
Traceback (most recent call last):
File "/opt/az/lib/python3.6/site-packages/knack/cli.py", line 215, in invoke
cmd_result = self.invocation.execute(args)
File "/opt/az/lib/python3.6/site-packages/azure/cli/core/commands/__init__.py", line 654, in execute
raise ex
File "/opt/az/lib/python3.6/site-packages/azure/cli/core/commands/__init__.py", line 718, in _run_jobs_serially
results.append(self._run_job(expanded_arg, cmd_copy))
File "/opt/az/lib/python3.6/site-packages/azure/cli/core/commands/__init__.py", line 709, in _run_job
cmd_copy.exception_handler(ex)
File "/home/daporo/.azure/cliextensions/azure-devops/azext_devops/dev/common/exception_handler.py", line 31, in azure_devops_exception_handler
reraise(*sys.exc_info())
File "/opt/az/lib/python3.6/site-packages/six.py", line 703, in reraise
raise value
File "/opt/az/lib/python3.6/site-packages/azure/cli/core/commands/__init__.py", line 688, in _run_job
result = cmd_copy(params)
File "/opt/az/lib/python3.6/site-packages/azure/cli/core/commands/__init__.py", line 325, in __call__
return self.handler(*args, **kwargs)
File "/opt/az/lib/python3.6/site-packages/azure/cli/core/__init__.py", line 545, in default_command_handler
return op(**command_args)
File "/home/daporo/.azure/cliextensions/azure-devops/azext_devops/dev/team/invoke.py", line 44, in invoke
request_body = json.loads(in_file_content)
File "/opt/az/lib/python3.6/json/__init__.py", line 354, in loads
return _default_decoder.decode(s)
File "/opt/az/lib/python3.6/json/decoder.py", line 339, in decode
obj, end = self.raw_decode(s, idx=_w(s, 0).end())
File "/opt/az/lib/python3.6/json/decoder.py", line 357, in raw_decode
raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

**Additional context**
Under https://github.com/Azure/azure-devops-cli-extension/blob/master/azure-devops/azext_devops/dev/team/invoke.py#L37 expects --in-file to be json only, but in some scenarios is not the case.