multipart/form-data file upload with 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?

I did try that approach actually. It generates a string that is too long. And I would like to use the code above (if possible) to make it match the C# sample I have too. — ThomasArdal
– ThomasArdal, Commented Aug 6, 2021 at 8:31
Last time I checked the Web cmdlets in PowerShell did not directly support mutlipart/form-data and most of the working examples create some kind of http template for the -Body parameter. 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. — Ash
– Ash, Commented Aug 6, 2021 at 8:47
You're right. PowerShell doesn't support the code I'm writing. And now I got my code working with the example you provided. Thank you so much. Feel free to publish in answer and I can mark it as the right answer. I can put my code in your answer once it is created. — ThomasArdal
– ThomasArdal, Commented Aug 6, 2021 at 9:12

Ash · Accepted Answer · 2022-10-25 10:30:56Z

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:

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