Powershell Invoke-RestMethod Smartsheet API add attachment to sheet - powershell

Question

I am trying to use the Invoke-RestMethod to upload an attachment to a Smartsheet using the API.
This is the curl example from the Smartsheet API Documentation:
curl https://api.smartsheet.com/2.0/sheets/{sheetId}/attachments \
-H "Authorization: Bearer ll352u9jujauoqz4gstvsae05" \
-H "Content-Type: application/msword" \
-H 'Content-Disposition: attachment; filename="ProgressReport.docx"' \
-H "Content-Length: FILE_SIZE" \
-X POST \
--data-binary #ProgressReport.docx
This is my Powershell code.
$BaseURI = "https://api.smartsheet.com/2.0"
$Id = {sheet id} #sheet id passed a parameter
$Uri = "{0}/sheets/{1}/attachments" -f $BaseUri, $id
$mimetype = ""application/vnd.openxmlformats-officedocument.wordprocessingml.document"
$FileName = "MyDocument.docx"
$Config = Read-Config #reads authentication token from text file"
$token = ConvertTo-SecureString -String $config.APIKey -AsPlainText -Force
$response = Invoke-RestMethod -Method POST -Uri $Uri -InFile $FileName -ContentType $mimetype -Authentication:Bearer -token $token
What I get is this response:
{ "errorCode" : 1008, "message" : "Unable to parse request. The following error occurred: Request body must be either a JSON
| object or JSON array.", "refId" : "12y60zzovzndm" }
If I try to post a text file with just the work "test" in it I get this error.
{ "errorCode" : 1008, "message" : "Unable to parse request. The following error occurred: Unrecognized token 'test': was expecting 'null', 'true', 'false' or NaN\n at [Source: REST input; line: 1, column: 6]", "refId" : "lbstw0mq73l9" }
If I do this API call in Postman it works fine.
I know there is a C# SDK but I'm building a PowerShell module and I don't want those .dll files as a requirement for this module.
OK I got this to work thanks to Postman.
I build the headers as a hashtable"
Name Value
---- -----
Content-Type application/vnd.openxmlformats-officedocument.wordprocessingml.document
Authorization Bearer VcjJ....
Content-Disposition attachment; filename="CWilliams.docx"
Convert the input file into a byte array with:
$body = [System.IO.File]::ReadAllBytes($path)
Then issue the statement:
$response = Invoke-RestMethod -Method 'POST' -Uri $Uri -Headers $Headers -Body $body
From what I understand about the Invoke-RestMethod function the -inFile parameter is supposed to handle all this for you but it doesn't work with the Smartsheet API.