What happens when for part called file, you specify $FileDocument, not $FileDocument/Contents?
if possible if you can provide the path of the file Stored ( Should be shared path ) , then the path is quite enough to share . The third party API method will access that path and get the document .
If no then you need to send a base64 Code to API and a file type , and conversion is taken care by service provider .
there is no other way to share the document .
If you are using any GET API , You can get directly doc in to the entity which is generalized with file document entity .