Post multi-part body with PowerShell Invoke-RestMethod - powershell

Question

Using Postman the API call works. I used their code snippet feature to get the PowerShell equivalent. But when I try it on PowerShell ISE I get "Empty request body not allowed". Why does it think the body is empty? How can I confirm/look at the contents of $body?
using assembly System.Net.Http
using namespace System.Net.Http
$headers = New-Object "System.Collections.Generic.Dictionary[[String],[String]]"
$headers.Add("Authorization", "Bearer token")
$multipartContent = [System.Net.Http.MultipartFormDataContent]::new()
$multipartFile = 'C:\Users\xxx\Desktop\msg.json'
$FileStream = [System.IO.FileStream]::new($multipartFile, [System.IO.FileMode]::Open)
$fileHeader = [System.Net.Http.Headers.ContentDispositionHeaderValue]::new("form-data")
$fileHeader.Name = "ParameterRequest"
$fileContent = [System.Net.Http.StreamContent]::new($FileStream)
$fileContent.Headers.ContentDisposition = $fileHeader
$multipartContent.Add($fileContent)
$FileStream.Flush()
$FileStream.Close()
$multipartFile = 'C:\Users\xxx\Desktop\edm_payload'
$FileStream = [System.IO.FileStream]::new($multipartFile, [System.IO.FileMode]::Open)
$fileHeader = [System.Net.Http.Headers.ContentDispositionHeaderValue]::new("form-data")
$fileHeader.Name = "MessagePayload"
$fileContent = [System.Net.Http.StreamContent]::new($FileStream)
$fileContent.Headers.ContentDisposition = $fileHeader
$multipartContent.Add($fileContent)
$FileStream.Flush()
$FileStream.Close()
$body = $multipartContent
$response = Invoke-RestMethod -Method Post -Uri 'https://APIsite' -Headers $headers -Body $body -ContentType "multipart/form-data"
When I look at the contents of $body this is what I get. How can I see the data in each Content?
PS C:\Users\user> $body
Headers
-------
{[Content-Disposition, System.String[]]}
{[Content-Disposition, System.String[]]}
Added -Verbose to the Invoke-RestMethod and I see
VERBOSE: POST https://APIsite with -1-byte payload
I cannot find what is the meaning of "-1-byte payload". Anybody knows?

Answer 1

Solved. Part of the problem was that I was using PowerShell v5, the MultipartFormDataContent has been improved since then. Updated to version 7 of PowerShell, made a few tweaks... Success!
$formContent = New-Object -TypeName 'System.Net.Http.MultipartFormDataContent'
$filePath = "C:\Users\xxx\Desktop\msg.json"
$fileStream1 = [System.IO.File]::Open($filePath, [System.IO.FileMode]::Open, [System.IO.FileAccess]::Read)
$formContent.Add([System.Net.Http.StreamContent]::new($fileStream1), "ParameterRequest", (Split-Path $filePath -leaf))
$filePath = "C:\Users\xxx\Desktop\edm_payload"
$fileStream2 = [System.IO.File]::Open($filePath, [System.IO.FileMode]::Open, [System.IO.FileAccess]::Read)
$formContent.Add([System.Net.Http.StreamContent]::new($fileStream2), "MessagePayload", (Split-Path $filePath -leaf))
Invoke-RestMethod -Method Post -Uri 'https://APIsite' -Headers $requestHeader -Body $formContent -ContentType 'multipart/form-data'
Also - close your streams after the API call