Upload Attachment through Force.com sites

UploadAttachment.page

<apex:page controller="UploadAttachmentController">
    <script type="text/javascript">
        function alertFilesize(){
            if(window.ActiveXObject){
                var fso = new ActiveXObject("Scripting.FileSystemObject");
                var filepath = document.getElementById('j_id0:j_id2:fileInput').value;
                var thefile = fso.getFile(filepath);
                var sizeinbytes = thefile.size;
            }else{
                var sizeinbytes = document.getElementById('j_id0:j_id2:fileInput').files[0].size;
            }
        
            var fSExt = new Array('Bytes', 'KB', 'MB', 'GB');
            fSize = sizeinbytes; i=0;while(fSize>900){fSize/=1024;i++;}
            
            alert(' File Size should be less than 10 MB. Your current file size is ' + (Math.round(fSize*100)/100)+' '+fSExt[i]);
        }
    </script>
    <apex:form>
        <apex:pageMessages id="msg"></apex:pageMessages>
        
        <div style="display:block; width: 10%; float:left; margin-bottom: 10px;">
            Choose a file 
        </div>
        <div style="display:block; width: 90%; float:left; margin-bottom: 10px;">
            <apex:inputFile id="fileInput" onchange="alertFilesize();" value="{!attBody}" fileName="{!fileName}" fileSize="{!fileSize}" contentType="{!contentType}"></apex:inputFile>
        </div>
        <div style="display:block; width: 10%; float:left; margin-bottom: 10px;">
        </div>
        <div style="display:block; width: 90%; float:left; margin-bottom: 10px;">    
            <apex:commandButton action="{!uploadFile}" value="Upload File"/>
        </div>
    </apex:form>
</apex:page>

UploadAttachmentController.cls

public class UploadAttachmentController {
    
    public Blob attBody { get; set; }
    public String fileName { get; set; }
    public Integer fileSize { get; set; }
    public String contentType { get; set; }
    private Id parentRecordId;
    
    public UploadAttachmentController(){
        
        parentRecordId = ApexPages.currentPage().getParameters().get('id');
        
    }
    
    public PageReference uploadFile() {
        
        if( attBody != null ) { 
            Attachment att = new Attachment( Body = attBody, 
                                         Name = fileName, 
                                         ParentId = parentRecordId,
                                         ContentType = contentType ); 
            try { 
                insert att;
                ApexPages.addMessage(new ApexPages.Message( ApexPages.severity.CONFIRM, 'File uploaded successfully.') );
            } catch ( Exception e ) {
                ApexPages.addMessage(new ApexPages.Message( ApexPages.severity.ERROR, 'File upload failed due to "' + e.getMessage() + '".') );
            }
            
            attBody = null;
            fileName = '';
        }
        else {
            ApexPages.addMessage(new ApexPages.Message( ApexPages.severity.ERROR, 'Please choose a file.') );
        }
        
        PageReference pg = Page.UploadAttachment;
        pg.setRedirect( false );
        return pg;
    }
}