RestApi Excel file upload (update) corrupts the file

I have the following PowerShell scrip which reads an .xlsx input file from the file system and uploads it to a page in Confluence:

Param
(
[Parameter(Mandatory=$true)] [string]$userName,
[Parameter(Mandatory=$true)] [string]$userPassword,
[Parameter(Mandatory=$true)] [string]$fileName,
[Parameter(Mandatory=$true)] [string]$filePath
)
$wpURL = "https://xxxxxxxxxxxxxxxxxxxx/rest/api/content/"
$secStringPassword = ConvertTo-SecureString $userPassword -AsPlainText -Force
$Headers = @{'Authorization' = "Basic "+[System.Convert]::ToBase64String([System.Text.Encoding]::UTF8.GetBytes(($userName+":"+[System.Runtime.InteropServices.marshal]::PtrToStringAuto([System.Runtime.InteropServices.marshal]::SecureStringToBSTR($secStringPassword)) )))
            'X-Atlassian-Token' = 'nocheck'
}

$pageId = "xxxxxxxxx"

$uri = $wpURL + $pageId + "/child/attachment?expand=body.storage,version,space,ancestors"
$listOfAttachments = Invoke-WebRequest -Method GET -Headers $Headers -Uri $uri  | ConvertFrom-Json
$listOfAttachments.results
foreach($result in $listOfAttachments.results){
    if($result.title -eq $fileName){
         $attachmentId = $result.id
    }
}

$fileBytes = [System.IO.File]::ReadAllBytes($filePath);
$fileEncode = [System.Text.Encoding]::GetEncoding('UTF-8').GetString($fileBytes);
$delimiter = [System.Guid]::NewGuid().ToString(); 
$LF = "`r`n";
$bodyData = ( 
    "--$delimiter",
    "Content-Disposition: form-data; name=`"file`"; filename=`"$fileName`"",
    "Content-Type: application/octet-stream$LF",
    $fileEncode,
    "--$delimiter--$LF" 
) -join $LF

Invoke-RestMethod -Uri $uri -Method Post -ContentType "multipart/form-data; boundary=`"$boundary`"" -Body $bodyLines
$attachmentIdUriPath = "/child/attachment/$attachmentId/data"
$uri = $ConfluenceURL + $pageId + "/child/attachment/" + $attachmentId.ToString() + "/data"
Invoke-RestMethod -Uri $uri -Method POST -ContentType "multipart/form-data; boundary=`"$delimiter`"" -Headers $Headers -Body $bodyData

It works to the extend that it does read and upload the file, the file is not empty and looks like an Excel file, I can download it from the web page, BUT I cannot open it as it is corrupt.

I also see this error while running it in PS:

Invoke-RestMethod : XSRF check failed
At C:\UiPath\Processes\ERP_ReleaseNotesUploadToClientWeb\Data\PSScript.ps1:38 char:1
+ Invoke-RestMethod -Uri $uri -Method Post -ContentType "multipart/form ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: (System.Net.HttpWebRequest:HttpWebRequest) [Invoke-RestMethod], WebException
    + FullyQualifiedErrorId : WebCmdletWebResponseException,Microsoft.PowerShell.Commands.InvokeRestMethodCommand

so, I have a tip that something is wrong with -ContentType in the last line, but have not figured out what on earth might be wrong.

Maybe some fresh eyes could help?

OK, strangely enough the corruption occurs because of the "UTF-8" encoding. It worked fine when I put "iso-8859-1" instead ...