Invoice Physical Attachment Error

I am trying to use a physical PDF in the Invoice Attachments API from Financial Edge NXT and I keep getting an error about converting the value to a file. I can do a link attachment, but we want to use a physical attachment of a pdf to a specific invoice. My request body is this:

{

"parent_id": 2156,

"name": "Test PDF Attachment",

"file": "C:/testfile.pdf",

"type": "Physical",

"media_type": "Other"

}


The error I keep getting is this:

{ "Error": "Invalid Request", "Details": ["Error converting value \\"C:/testfile.pdf\\" to type 'System.Web.HttpPostedFileBase'. Path 'file', line 4, position 27."], "ErrorPath": null }


I have tried everything I can think of and I still get that error. Any guidance would be greatly appreciated.