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