How to send multipart/form-data with PowerShell Invoke-RestMethod

The accepted answer won't do a multipart/form-data request, but rather a application/x-www-form-urlencoded request forcing the Content-Type header to a value that the body does not contain.

One way to send a multipart/form-data formatted request with PowerShell is:

$ErrorActionPreference = 'Stop'

$fieldName = 'file'
$filePath = 'C:\Temp\test.pdf'
$url = 'http://posttestserver.com/post.php'

Try {
    Add-Type -AssemblyName 'System.Net.Http'

    $client = New-Object System.Net.Http.HttpClient
    $content = New-Object System.Net.Http.MultipartFormDataContent
    $fileStream = [System.IO.File]::OpenRead($filePath)
    $fileName = [System.IO.Path]::GetFileName($filePath)
    $fileContent = New-Object System.Net.Http.StreamContent($fileStream)
    $content.Add($fileContent, $fieldName, $fileName)

    $result = $client.PostAsync($url, $content).Result
    $result.EnsureSuccessStatusCode()
}
Catch {
    Write-Error $_
    exit 1
}
Finally {
    if ($client -ne $null) { $client.Dispose() }
    if ($content -ne $null) { $content.Dispose() }
    if ($fileStream -ne $null) { $fileStream.Dispose() }
    if ($fileContent -ne $null) { $fileContent.Dispose() }
}

The problem here was what the API required some additional parameters. Initial request required some parameters to accept raw content and specify filename/size. After setting that and getting back proper link to submit, I was able to use:

Invoke-RestMethod -Uri $uri -Method Post -InFile $filePath -ContentType "multipart/form-data"

I found this post and changed it a bit

$fileName = "..."
$uri = "..."

$currentPath = Convert-Path .
$filePath="$currentPath\$fileName"

$fileBin = [System.IO.File]::ReadAlltext($filePath)
$boundary = [System.Guid]::NewGuid().ToString()
$LF = "`r`n"
$bodyLines = (
    "--$boundary",
    "Content-Disposition: form-data; name=`"file`"; filename=`"$fileName`"",
    "Content-Type: application/octet-stream$LF",
    $fileBin,
    "--$boundary--$LF"
) -join $LF

Invoke-RestMethod -Uri $uri -Method Post -ContentType "multipart/form-data; boundary=`"$boundary`"" -Body $bodyLines