Upload a file to Box.com using Powershell

I am working on a series of Powershell scripts for my company to use to transfer data to and from Box.com. The one thing I just can't figure out is uploading files. The Box API requires a multipart POST operation for uploads, and I've seen a few answers here on SO indicating that I should be able to do that in Powershell (such as this one). But I can't seem to get it working.

Here's the code I have right now:

Function Post-File {
    Param(
            [Parameter(Mandatory=$True,Position=1)]
            [string]$SourcePath,
            [Parameter(Mandatory=$False,Position=2)]            
            [string]$FolderId = ############
    )

    #Variables for building URIs
    $baseUrl = "https://upload.box.com/api/2.0/files/content"

    #Set Authorization for API requests
    $headers = @{}
    $AccessToken = Refresh-Tokens #A reference to another function that definitely works
    $headers.Add("Authorization", "Bearer $AccessToken")

    #Set POST content
    $body = @{}
    $body.Add("filename",  [IO.File]::ReadAllBytes($SourcePath))
    $body.Add("parent_id", $FolderId)

    #Upload the file
    Invoke-RestMethod -Uri $baseUrl -Method Post -Headers $headers -ContentType "multipart/form-data" -Body $body
}

Here's the response I get back:

{
 "type":"error",
 "status":400,
 "code":"invalid_request_parameters",
 "help_url":"http://developers.box.com/docs/#errors",
 "message":"Invalid input parameters in request",
 "request_id":"1764475572534bcddfe04b7"
}

I've also tried a couple of other permutations that aren't working. I've tried using the -InFile switch in Invoke-RestMethod instead of -Body. I've also tried using Get-Content -Raw in place of [IO.File]::ReadAllBytes. Both of those return a more generic error: The server encountered an internal error or misconfiguration and was unable to complete your request..

I'm pretty sure this has something to do with my filename parameter not being correct, but I'm not sure how to fix it. According to the Box API, here's what it should look like in curl. Can someone help me properly translate this for Powershell?

https://upload.box.com/api/2.0/files/content \
-H "Authorization: Bearer ACCESS_TOKEN" \
-F filename=@FILE_NAME \
-F parent_id=PARENT_FOLDER_ID

There are a couple things that make this a little tricky in PowerShell:

1. The filename parameter specifies the name of the file, not the contents.
2. A request body is required in order to specify the file name and destination.
3. PowerShell treats -InFile and -Body arguments as mutually exclusive.
4. PowerShell does not appear to natively support multipart/form-data POSTs, per the question that you referenced.

Since the request body is required (1,2) -- and thus -InFile can't be used (3) -- I think you might need to roll your own multipart/form-data body (4) that contains the necessary metadata and file content. This blog post describes a method for doing so. The content there is a short string (a tweet) but the principle is the same.

Below is a Fiddler trace of an upload I just performed using the Box Windows SDK. This shows how the request should look as it goes across the wire. The $BOUNDARY is an arbitrary, unique string -- a GUID works great.

POST https://upload.box.com/api/2.0/files/content HTTP/1.1
Authorization: Bearer $TOKEN
Content-Type: multipart/form-data; boundary="$BOUNDARY"
Host: upload.box.com
Content-Length: 2118
Accept-Encoding: gzip, deflate

--$BOUNDARY

Content-Disposition: form-data; name="file"; filename="$FILENAME"

<THE CONTENT OF $FILENAME>    

--$BOUNDARY

Content-Type: text/plain; charset=utf-8
Content-Disposition: form-data; name="metadata"

{"parent":{"id":"$PARENT_FOLDER_ID"},"name":"$FILENAME"}

--$BOUNDARY--

Here is the response I received:

HTTP/1.1 201 Created
Date: Mon, 14 Apr 2014 12:52:33 GMT
Server: Apache
Cache-Control: no-cache, no-store
Content-Length: 364
Connection: close
Content-Type: application/json

{"total_count":1,"entries":[{"type":"file","id":"$ID","name":"$FILENAME", ... }]}