ConvertTo-MultipartFormData.ps1

<#
.SYNOPSIS
Creates multipart/form-data to send as a request body.

.INPUTS
Any System.Collections.IDictionary type of key-value pairs to encode.

.OUTPUTS
System.Byte[] of encoded key-value data.

.FUNCTIONALITY
HTTP

.LINK
https://docs.microsoft.com/dotnet/api/system.net.http.multipartformdatacontent

.LINK
Invoke-WebRequest

.LINK
Invoke-RestMethod

.LINK
New-Guid

.EXAMPLE
@{ title = 'Name'; file = Get-Item avatar.png } |ConvertTo-MultipartFormData.ps1 |Invoke-WebRequest $url -Method POST

Sends two fields, one of which is a file upload.
#>

#Requires -Version 3
[CmdletBinding()][OutputType([byte[]])] Param(
<#
The fields to pass, as a Hashtable or other dictionary.
Values of the System.IO.FileInfo type will be read, as for a file upload.
#>
[Parameter(Mandatory=$true,Position=0,ValueFromPipeline=$true)][Collections.IDictionary] $Fields
)
DynamicParam
{
	$boundary = "$(New-Guid)"
	$PSDefaultParameterValues['Invoke-WebRequest:ContentType'] = "multipart/form-data; boundary=$boundary"
	$PSDefaultParameterValues['Invoke-RestMethod:ContentType'] = "multipart/form-data; boundary=$boundary"
}
Begin
{
	$cmdletname = $MyInvocation.MyCommand.Name
	if((Get-Module Microsoft.PowerShell.Utility).Version -ge [version]6.1)
	{
		Write-Warning "Invoke-WebRequest and Invoke-RestMethod appear to natively support multipart/form-data."
	}
	try{[void][Net.Http.StringContent]}catch{Add-Type -AN System.Net.Http |Out-Null}
}
Process
{
	$content = New-Object Net.Http.MultipartFormDataContent $boundary
	foreach($field in $Fields.GetEnumerator())
	{
		if($field.Value -isnot [IO.FileInfo])
		{
			$content.Add([Net.Http.StringContent]$field.Value,$field.Key)
			Write-Verbose "$cmdletname : $($field.Key)=$($field.Value)"
		}
		else
		{
			Write-Verbose "$cmdletname : Adding file $($field.Value.FullName)"
			$content.Add((New-Object Net.Http.StreamContent ($field.Value.OpenRead())),$field.Key,$field.Value.Name)
		}
	}
	$content.Headers.GetEnumerator() |
		ForEach-Object {Write-Verbose "$cmdletname header : $($_.Key)=$($_.Value -join "`n`t")"}
	[Threading.Tasks.Task[byte[]]]$getbody = $content.ReadAsByteArrayAsync()
	$getbody.Wait()
	[byte[]]$body = $getbody.Result
	$content.Dispose()
	Write-Verbose "$cmdletname : Body is $($body.Length) bytes"
	return,$body
}
End
{
	$PSDefaultParameterValues.Remove('Invoke-WebRequest:ContentType')
	$PSDefaultParameterValues.Remove('Invoke-RestMethod:ContentType')
}