Hi nihar,
Here is a solution to your Problem.

                ----------Trigger--------
                
trigger InsertFiletrigger on Opportunity (after insert) {
    InsertFile.fileInsertion(trigger.new); 
}

                ----------Trigger_Handler-------
                
public with sharing class InsertFile {
    public static void fileInsertion(List<Opportunity> lstOpp){
        Set<Id> stAccId = new Set<Id>();
        for(Opportunity opp: lstOpp){
            stAccId.add(opp.AccountId);
        }
        System.debug('List of AccountId>>>>>>'+ stAccId);
        List<Opportunity> lstAllOpp = new List<Opportunity>();
        lstAllOpp = [SELECT Id,Name,AccountId FROM Opportunity Where AccountId IN:stAccId];
        System.debug('ListAllOpp >>>>>>'+ lstAllOpp);
        
        List<Attachment> lstAttachment = new List<Attachment>();
        lstAttachment = [SELECT Id,ParentId,Name,Body FROM Attachment Where ParentId IN: lstAllOpp];
        System.debug('ListAttach >>>>>>'+ lstAttachment);
        
        List<Attachment> lstToAddAttch = new List<Attachment>();
        for(Opportunity opp: lstOpp){
            for(Attachment attach : lstAttachment) {
                Attachment objattach =new Attachment();
                objattach.body = attach.body;
                objattach.name = attach.name;
                objattach.ParentId = opp.Id;
                lstToAddAttch.add(objattach);
            }
        }
        System.debug('ListToAttach >>>>>>'+ lstToAddAttch);
        if(lstToAddAttch.size()>0)
            insert lstToAddAttch;
    }
}


Please mark this as Best Answer if you find this solution helpful.

Thank You
Ajay Dubedi​