Hi,
In this case once record will save successfully then rendered "Attach a File" button which should be overriden by VF page.
you can use teh concept from below sample page and place your logic within extension class.Hope this will help..

<apex:page id="uploadImagePage" standardController="Contact" extensions="Contactsextension"> <style type="text/css"> .pic { background-color : #F3F3EC; border:2px solid #E3DEB8; height:100px; /*margin :15px auto;*/ padding:5px; width:100px; margin:20px; } .buttons , .input { margin:20px; } </style> <script> function verifyNameLength(){ var textVal = document.getElementsByName('uploadImagePage:uploadAttachment:newAttach:inputFile:file')[0].value; if( textVal != "" ) { var browser = navigator.userAgent; var fileName = textVal; if( browser.search( 'MSIE' ) > 0 ) { var index = textVal.lastIndexOf( '\\' ); fileName = textVal.substring( index +1); } if( fileName.length < 70) { return true; }; }; if( textVal == "" ) { alert( 'Please select an image to upload' ); } else { alert( 'The file name must be 70 characters maximum' ); } document.getElementById('uploadImagePage:uploadAttachment').reset(); return false; } </script> <apex:form id="uploadAttachment"> <apex:outputpanel styleClass="pic" id="image" layout="block"> <apex:image value="{!URLFOR($Resource.PictureUploader, '/images/unknownperson.jpg')}" rendered="{!NOT( hasPicture )}" height="100" width="100"/> <apex:image value="/servlet/servlet.FileDownload?file={!file.Id}" rendered="{!hasPicture}" height="100" width="100"/> </apex:outputpanel> <div class="input"> <b>1. Select the File</b> : Type the path of the file or click the Browse button to find the file.<br/> <apex:inputFile value="{!newAttach.Body}" id="newAttach" styleclass="newAttach" contentType="{!newAttach.ContentType}" filename="{!newAttach.Name}" fileSize="{!newAttach.BodyLength}" /> <br/> <apex:outputText value="{!error}" escape="false" styleClass="errorMsg"/> </div> <div class="buttons"> <b>2. Click the "Upload" button or "Delete" button in order to delete the current picture </b><br/> <apex:commandButton id="Accept" action="{!uploadAction}" value="Upload" onclick="return verifyNameLength();"></apex:commandButton> <apex:commandButton id="Delete" action="{!deleteAction}" value="Delete" rendered="{!hasPicture}" onclick="return confirm('Are you sure you want to delete the current image?')"></apex:commandButton> <apex:commandButton id="Cancel" action="{!cancel}" value="Cancel"></apex:commandButton> </div> </apex:form> </apex:page>

within extension use below methods for upload /save/ delete actions.

public PageReference uploadAction(){
        
        PageReference thePage = new PageReference( '/'+ parentId );
        thePage.setRedirect( true );
        if( this.validate() ){
            return ( this.saveCurrentPicture() ) ? thePage : null;
        }
        else{
            this.newAttach = new Attachment();
            return null;
        }
    }
    
    

    public Boolean saveCurrentPicture(){
        Savepoint sp = Database.setSavepoint();
        try{
            delete [ Select Id From Attachment where ParentId =: this.parentId and name = 'Contact Picture' limit 1 ];
            this.newAttach.parentId = this.parentId;
            this.newAttach.name = 'Contact Picture';
            insert this.newAttach;
            return true;
        } 
        catch( Exception e ){
            this.error += ERROR_NO_SAVE+'<br/>';
            Database.rollback( sp );
            return false;
        }
    }

    public PageReference deleteAction(){
        
        PageReference thePage = new PageReference( '/'+ parentId );
        thePage.setRedirect( true );
        delete this.file;
        return thePage;
    }

Thanks
Swarup