Solved - For anyone that also comes across this issue
Postman was not adding the content type of the file when parsing the code into powershell.
Add this for the XML fields file
$fileContent.Headers.Add("Content-Type", "application/xml")
And i did this to detect the actual document mimetype and set the header
Add-Type -AssemblyName "System.Web"
$Mime = [System.Web.MimeMapping]::GetMimeMapping($File)
$fileContent.Headers.Add("Content-Type", $Mime)