In the below example Notification will be sent to the Account owner when the file is attached to their Account records.

Notification Type in Setup:

Sample Code:

trigger ContentDocumentLinkTrigger on ContentDocumentLink ( after insert ) {
    
       
    Set < Id > setAccIds = new Set < Id >();
    Map < Id, Id > mapAccIdContDocId = new Map < Id, Id >();
       
    for ( ContentDocumentLink objCDL : trigger.new ) {
       
        String strEntityId = objCDL.LinkedEntityId;
        
        if ( String.isNotBlank( objCDL.LinkedEntityId ) && strEntityId.left( 3 ) == '001' ) {
       
            setAccIds.add( objCDL.LinkedEntityId );
            mapAccIdContDocId.put( objCDL.LinkedEntityId, objCDL.ContentDocumentId );
           
        }
        
    }
    
    if ( setAccIds.size() > 0 ) {
    
        CustomNotificationType notificationType = [SELECT Id FROM CustomNotificationType WHERE DeveloperName = 'Desktop'];
                   
        Messaging.CustomNotification notification = new Messaging.CustomNotification();
        Set < String > recipientsIds = new Set < String >();
        notification.setTitle( 'File Attached to your Account' );
        notification.setNotificationTypeId( notificationType.Id );
    
        for ( Account objAcc : [ SELECT Id, OwnerId FROM Account WHERE Id IN: setAccIds ] ) {
       
            notification.setBody( 'File Id is ' + mapAccIdContDocId.get( objAcc.Id ) );
            notification.setTargetId( objAcc.Id );
            recipientsIds.add( objAcc.OwnerId );
            notification.send( recipientsIds );
       
        }
    
    }

}

Output:

send() is not bulkified. So, remember 150 DML Governor limit.

https://trailblazer.salesforce.com/ideaView?id=0874V0000010y6zQAA