Proxy does not foward custom HTTP headers

[albtrentadue]

Hello.
We are using ToxiProxy for negative testing our application.
One of our endpoints returns a file stream over http and puts 2 information in headers:
"Content-Length" : this is a commonly used http header
"File-Checksum" : this is our own application specific header

When requesting a file throw ToxiProxy, the "File-Checksum" header is missing and we can see these two headers:
"Content-Length"
"Date"

Question: is this an expected behaviour of ToxiProxy? If yes, could it be useful to keep the headers as is without changes?

Thank you for advice