SharePoint Online: Add Attachment to List Item using PowerShell

Requirement: Add Attachment to SharePoint Online List Item

SharePoint Online: How to Add Attachment to List Item?
To add attachment to list item in SharePoint Online,

• Navigate to the SharePoint Online list >> Select the List Item you want to add attachments
• Edit List Item >> Scroll down and Click on "Add Attachments" link in the Edit pane.
  
• This opens browse dialog window., Select attachments to add
• Click on Save to complete adding attachment to list item.

PowerShell to Add an Attachment to List Item:

#Load SharePoint CSOM Assemblies
Add-Type -Path "C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\16\ISAPI\Microsoft.SharePoint.Client.dll"
Add-Type -Path "C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\16\ISAPI\Microsoft.SharePoint.Client.Runtime.dll"

Function Add-AttachmentToListItem()
{
    param
    (
        [Parameter(Mandatory=$true)] [string] $SiteURL,
        [Parameter(Mandatory=$true)] [string] $ListName,
        [Parameter(Mandatory=$true)] [string] $ItemID,
        [Parameter(Mandatory=$false)] [string] $AttachmentPath
    )    
    Try {
        #Setup Credentials to connect
        $Cred = Get-Credential
        $Cred = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($Cred.UserName,$Cred.Password)
    
        #Setup the context
        $Ctx = New-Object Microsoft.SharePoint.Client.ClientContext($SiteURL)
        $Ctx.Credentials = $Cred

        #Get the List & List Item
        $List = $Ctx.Web.Lists.GetByTitle($ListName)
        $Ctx.Load($List)
        $ListItem = $List.GetItemByID($ItemID)
        $Ctx.Load($ListItem)
        $Ctx.ExecuteQuery()

        #Get All existing attachments
        $AttachmentFiles = $ListItem.AttachmentFiles
        $Ctx.Load($AttachmentFiles)
        $Ctx.ExecuteQuery()

        #Check if attachment file name exists already
        $FileName = Split-Path $AttachmentPath -Leaf
        $AttachmentFile = $AttachmentFiles | where { ($_.FileName -eq $FileName) }
        If($AttachmentFile -eq $Null)
        {
            #Get the Attachment file from local disk
            [Byte[]]$Bytes = [System.IO.File]::ReadAllBytes($AttachmentPath)
            $ContentStream = New-Object -TypeName System.IO.MemoryStream -ArgumentList @(,$Bytes)
    
            #Create Attachment object
            $AttachmentCreation = New-Object Microsoft.SharePoint.Client.AttachmentCreationInformation
            $AttachmentCreation.ContentStream = $ContentStream
            $AttachmentCreation.FileName = $FileName
            [Void]$ListItem.AttachmentFiles.Add($AttachmentCreation)
            $Ctx.ExecuteQuery()

            write-host  -f Green "Attachment Added to List Item!" 
        }
        else
        {
            write-host -f Yellow "Attachment File Name Exists already!"
        }
    }
    Catch {
        write-host -f Red "Error Adding Attachment to List!" $_.Exception.Message
    }
}

#Set Parameters
$SiteURL= "https://crescent.sharepoint.com"
$ListName="Projects"
$ItemID="5"
$AttachmentPath="C:\Projects\Proposal.docx"

#Call the function to copy list items
Add-AttachmentToListItem -SiteURL $SiteURL -ListName $ListName -ItemID $ItemID -AttachmentPath $AttachmentPath

PnP PowerShell to Add Attachment to List Item 
Let's add an attachment to the SharePoint Online list item with PnP PowerShell.

#Set Variables
$SiteURL = "https://crescent.sharepoint.com/sites/Marketing"
$ListName = "Projects"
$ItemID ="2"
$FilePath ="C:\Docs\UserInfo.csv"

#Connect to PNP Online
Connect-PnPOnline -Url $SiteURL -Credentials (Get-Credential)

#Get the List Item
$Item  = Get-PnPListItem -List $ListName -Id $ItemID

#Get the File and Add to List Item Attachment
$FileStream = New-Object IO.FileStream($FilePath,[System.IO.FileMode]::Open) 
$AttachmentInfo = New-Object -TypeName Microsoft.SharePoint.Client.AttachmentCreationInformation 
$AttachmentInfo.FileName =  Split-Path $FilePath -Leaf 
$AttachmentInfo.ContentStream = $FileStream
$AttchedFile = $Item.AttachmentFiles.Add($AttachmentInfo) 
Invoke-PnPQuery
$FileStream.Close()

How about adding multiple attachments to the list item? Lets add all files from a given folder to SharePoint Online.

#Set Variables
$SiteURL = "https://crescent.sharepoint.com/sites/pmo"
$ListName = "Projects"
$ItemID ="2"
$FolderPath = "C:\Temp\Docs"

#Connect to PNP Online
Connect-PnPOnline -Url $SiteURL -UseWebLogin

#Get the List Item
$ListItem  = Get-PnPListItem -List $ListName -Id $ItemID

#Function to Add all files as a attachment to list item
Function Add-AttachmentsFromFolder($ListItem, $FolderPath)
{
    #Get All existing attachments from list item
    $AttachmentFiles = Get-PnPProperty -ClientObject $ListItem -Property "AttachmentFiles"

    #Get the File and Add to List Item Attachment
    ForEach ($File in  (Get-ChildItem $FolderPath -File))
    {
        $AttachmentFile = $AttachmentFiles | Where { ($_.FileName -eq $File.Name) }
        If($AttachmentFile -eq $Null)
        {
            $AttachmentInfo = New-Object -TypeName Microsoft.SharePoint.Client.AttachmentCreationInformation 
            $AttachmentInfo.FileName =  $File.Name
            $AttachmentInfo.ContentStream = $File.OpenRead()
            $AttchedFile = $ListItem.AttachmentFiles.Add($AttachmentInfo) 
            Invoke-PnPQuery
            Write-host -f Green "Added Attachment File:"$File.FullName
        }
        Else
        {
            write-host -f Yellow "Attachment '$($File.Name)' Exists already!"
        }
    }
}
#Call the function to add all attachments from folder
Add-AttachmentsFromFolder $ListItem $FolderPath