Error 400 Script PowerShell? This script try to import a Dataflow in a Workspace

$credential = New-Object -TypeName System.Management.Automation.PSCredential -ArgumentList "...", ("..." | ConvertTo-SecureString -AsPlainText -Force)

Connect-PowerBIServiceAccount -ServicePrincipal -Tenant "..." -Credential $credential -Environment "Public"

$workspaceName = "WKS - DTS [DEV]"

$DataflowImportFileName = "C:\Users\cbonier\Documents\DTF FINANCE.json"

$DataflowDefinition = [IO.File]::ReadAllText($DataflowImportFileName)


$UserAccessToken = Get-PowerBIAccessToken
$bearer = $UserAccessToken.Authorization.ToString()


$workspace = Get-PowerBIWorkspace -Name $workspaceName
$workspaceId = $workspace.Id

$importsUrl = "https://api.powerbi.com/v1.0/myorg/groups/$workspaceId/imports?datasetDisplayName=DTF FINANCE.json"

$boundary = [System.Guid]::NewGuid().ToString("N")
$LF = [System.Environment]::NewLine

$contentType = "multipart/from-data; boundary=""$boundary"""

$body = (
"--$boundary",
"Content-Disposition: form-data $LF",
$DataflowDefinition,
"--$boundary--$LF"
) -join $LF

$header = @{
    'Authorization' = "$bearer"
    'Content-Type' = "$contentType"
}

Invoke-RestMethod -Uri $importsUrl -ContentType $contentType -Method Post -Headers $header -Body $body

Error :

Invoke-RestMethod : Le serveur distant a retourné une erreur : (400) Demande incorrecte.
Au caractère Ligne:38 : 1
+ Invoke-RestMethod -Uri $importsUrl -ContentType $contentType -Method  ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation : (System.Net.HttpWebRequest:HttpWebRequest) [Invoke-RestMethod], WebException
    + FullyQualifiedErrorId : WebCmdletWebResponseException,Microsoft.PowerShell.Commands.InvokeRestMethodCommand

This script aims to import a dataflow into a workspace in Power BI Service.