Hey @BostonJames,
Postman is a bit easier 
we have a community post from Dipro going over doing this through Postman. Essentially you would need send your data as a multipart request (Content-Type: multipart/form-data) with the file in a variable.
This actually brings another thing to mind on the PHP side of things - I think it would be a good idea to use CURL in this case to send the file. Using the same multipart/form-data content type as above, I think you would be able to send these using CURL. There is a StackOverflow thread discussing this approach.
Hopefully this helps! Let me know if you have any other questions.
-Daniel