var provider = new MultipartMemoryStreamProvider();
await Request.Content.ReadAsMultipartAsync(provider);

foreach (var file in provider.Contents)
{
    var filename = file.Headers.ContentDisposition.FileName.Trim('"');
    var buffer = await file.ReadAsByteArrayAsync();
    // Doing stuff with the file here
}

$apiKey=$args[0]
Write-Host "Using API Key: $apiKey"
$fileId=$args[1]
Write-Host "Using File ID: $fileId"
$sourceFile=$args[2]
Write-Host "Using File: $sourceFile"

$baseUrl = "http://localhost:23857/api"
$url = "$baseUrl/file/$fileId/version"

$fileBin = [IO.File]::ReadAllBytes($sourceFile)
$computer= $env:COMPUTERNAME
$enc = [System.Text.Encoding]::GetEncoding("iso-8859-1")
$fileEnc = $enc.GetString($fileBin)
$boundary = [System.Guid]::NewGuid().ToString()

$LF = "`n"
$bodyLines = (
"--$boundary",
"Content-Disposition: form-data; name=`"filename.txt`"$LF",   # filename= is optional
$fileEnc,
"--$boundary",
"Content-Disposition: form-data; name=`"computer`"$LF",
$computer,
"--$boundary--$LF"
) -join $LF

try {
    # Returns the response gotten from the server (we pass it on).
    #
    Invoke-RestMethod -Uri $url -Method Post -ContentType "multipart/form-data; boundary=`"$boundary`"" -TimeoutSec 120 -Body $bodyLines -Headers @{"ApiKey"=$apiKey}
}
catch [System.Net.WebException] {
    Write-Error( "FAILED to reach '$URL': $_" )
    throw $_
}

Unexpected end of MIME multipart stream. MIME multipart message is not complete.