multipart/form-data file upload with PowerShell - powershell

Question

I'm trying to upload a file to an ASP.NET Core API using PowerShell. My problem is that the API returns a status code 400 saying that form values are missing. Here's my PowerShell code:
add-type -AssemblyName System.Net.Http
$boundary = [System.Guid]::NewGuid().ToString()
$multipartContent = [System.Net.Http.MultipartFormDataContent]::new($boundary)
$multipartContent.Headers.Remove("Content-Type")
$multipartContent.Headers.TryAddWithoutValidation("Content-Type", "multipart/form-data; boundary=`"$boundary`"")
$stringContent = [System.Net.Http.StringContent]::new("This is a file I'm uploading")
$multipartContent.Add($stringContent, "Description")
$FilePath = "c:\path\to\file.json"
$FileStream = [System.IO.File]::OpenRead($FilePath)
$streamContent = [System.Net.Http.StreamContent]::new($FileStream)
$streamContent.Headers.ContentType = "application/json"
$multipartContent.Add($streamContent, "TheFile", "file.json")
$body = $multipartContent
$response = Invoke-RestMethod 'https://myapi.com/uploadfile' -Method 'POST' -Body $body
The code is pretty much migrated from a C# sample I have and is working. Can anyone see what I am doing wrong there?

Answer 1

As per my comment above, the Web cmdlets in PowerShell did not directly support mutlipart/form-data last I checked and most of the working examples create some kind of http template for the -Body parameter.
Here's a working sample:
$boundary = [System.Guid]::NewGuid().ToString()
$FilePath = "c:\path\to\file.json"
$TheFile = [System.IO.File]::ReadAllBytes($FilePath)
$TheFileContent = [System.Text.Encoding]::GetEncoding('iso-8859-1').GetString($TheFile)
$LF = "`r`n"
$bodyLines = (
"--$boundary",
"Content-Disposition: form-data; name=`"Description`"$LF",
"This is a file I'm uploading",
"--$boundary",
"Content-Disposition: form-data; name=`"TheFile`"; filename=`"file.json`"",
"Content-Type: application/json$LF",
$TheFileContent,
"--$boundary--$LF"
) -join $LF
Invoke-RestMethod 'https://myapi.com/uploadfile' -Method POST -ContentType "multipart/form-data; boundary=`"$boundary`"" -Body $bodyLines
Your other option if you want to keep it as close as possible to the C# code you have is to use HttpClient or whatever you are currently using instead of the web cmdlets in PowerShell.
Update: PowerShell 7+ does include a -Form parameter on both Web Cmdlets. Please see the linked examples from the official documentation:
Invoke-RestMethod
Invoke-WebRequest