It looks like you are just sending a path not the actual contents of a file, Does the API you are using have access to your local machine to be able to read the file or is it expecting you to actually send the file?
But you are still just sending the path not the file contents, I suspect when you use a URL the system you are working with is calling that URL to download it. When you are sending a filepath it looks like it is trying to read the local path on the server which is interesting and could potentially be a security issue if it is just blindly trying to read something… anyway…
Depending on what the API actually requires before the HTTP Request node add a read binary files node and put in the path to get the file data then send that to the service instead.