dropbox-upload.ps1

<#

.SYNOPSIS
This is a Powershell script to upload a file to DropBox using their REST API.

.DESCRIPTION
This Powershell script will upload file to DropBox using their REST API with the parameters you provide.

.PARAMETER SourceFilePath
The path of the file to upload.
.PARAMETER TargetFilePath
The path of the file on DropBox.

.ENV PARAMETER DropBoxAccessToken
The DropBox access token.
#>

Param(
    [Parameter(Mandatory=$true)]
    [string]$SourceFilePath,
    [Parameter(Mandatory=$true)]
    [string]$TargetFilePath
)

$arg = '{ "path": "' + $TargetFilePath + '", "mode": "add", "autorename": true, "mute": false }'
$authorization = "Bearer " + (get-item env:DropBoxAccessToken).Value

$headers = New-Object "System.Collections.Generic.Dictionary[[String],[String]]"
$headers.Add("Authorization", $authorization)
$headers.Add("Dropbox-API-Arg", $arg)
$headers.Add("Content-Type", 'application/octet-stream')
 
Invoke-RestMethod -Uri https://content.dropboxapi.com/2/files/upload -Method Post -InFile $SourceFilePath -Headers $headers

It happens to me too

Thanks. It worked for me.
My only issue was that I first had to create a "Dropbox app", and issue rights to it, to create the access token.
You need to assign rights (files.content.write) before you create the token, as the rights are stored inside the token !
Hope this helps.