converter with stdin

[ostrolucky]

Is your feature request related to a problem? Please describe.
This is a continuation from #1226 It looks like converter.command script gets format name and file name passed in only. However, in my case I'm operating with stdin.

Looks like there is no way to go from here. I checked arguments, env vars.

Describe the solution you'd like
Pass stdin through the command

Additional context
Add any other context or screenshots about the feature request here.