multipartmimestreamformatexception error when importing pbix files using rest api

hi,

So I'm using powershell to automate my power BI reports into my workspace through imports API.

here's the code which I'm using :

    $PBIXuri="my azure blob storage url",
    $Workspaceurl="https://api.powerbi.com/v1.0/myorg/groups/mygroups/imports?datasetDisplayName=myreportname",
    $AccessToken="my oauth token"

    #download my pbix file from azure blob storage
    $cli = New-Object System.Net.WebClient
    $exportpbix = $cli.DownloadData($PBIXuri)

    $powerBiBody = @'
    --exptest
    Content-Disposition: form-data; name="file";
    Content-Type: application/x-zip-compressed

    {0}
    --exptest--

    '@

    #set my pbix file into request body
    $encoding = [System.Text.Encoding]::GetEncoding('iso-8859-1')
    $body = $powerBiBody -f $encoding.GetString($exportpbix)

    $headers = @{
    "Authorization" = "Bearer " + $AccessToken}

    Invoke-RestMethod -Uri $Workspaceurl -Method Post -Headers $headers -Body $body -ContentType "multipart/form-data; boundary=--exptest"

but it always return an error like :

Invoke-RestMethod :     {"error":
     {"code":"MultiPartMimeStreamFormatException","pbi.error": 
       {"code":"MultiPartMimeStreamFormatException",
        "parameters":{},
        "details":[],
        "exceptionCulprit":1
        }
      }
    }

where do I went wrong on my code?