Uploading a Document using Visualforce and a Custom Controller

<apex:page controller="FileUploadCls">
  <apex:sectionHeader title="Visualforce Example" subtitle="File Upload Example"/>

  <apex:form enctype="multipart/form-data">
    <apex:pageMessages />
    <apex:pageBlock title="Upload a File">

      <apex:pageBlockButtons >
        <apex:commandButton action="{!fileupload}" value="Save"/>
      </apex:pageBlockButtons>

      <apex:pageBlockSection showHeader="false" columns="2" id="block1">

        <apex:pageBlockSectionItem >
          <apex:outputLabel value="File Name" for="fileName"/>
          <apex:inputText value="{!document.name}" id="fileName"/>
        </apex:pageBlockSectionItem>

        <apex:pageBlockSectionItem >
          <apex:outputLabel value="File" for="file"/>
          <apex:inputFile value="{!document.body}" filename="{!document.name}" id="file"/>
        </apex:pageBlockSectionItem>

        <apex:pageBlockSectionItem >
          <apex:outputLabel value="Description" for="description"/>
          <apex:inputTextarea value="{!document.description}" id="description"/>
        </apex:pageBlockSectionItem>

        <apex:pageBlockSectionItem >
          <apex:outputLabel value="Keywords" for="keywords"/>
          <apex:inputText value="{!document.keywords}" id="keywords"/>
        </apex:pageBlockSectionItem>

      </apex:pageBlockSection>

    </apex:pageBlock>
  </apex:form>
</apex:page>

Public class FileUploadCls{
    public Document document{    get{if(document==null)        document=new Document();        return document;    }    Set;    }   
    public PageReference fileupload() {

    document.AuthorId = UserInfo.getUserId();    document.FolderId = UserInfo.getUserId();      try {
      insert document;
    } catch (DMLException e) {
      ApexPages.addMessage(new ApexPages.message(ApexPages.severity.ERROR,'Error uploading file'));
      return null;
    } finally {
      document.body = null; // clears the viewstate      document = new Document();
    }

    ApexPages.addMessage(new ApexPages.message(ApexPages.severity.INFO,'File uploaded successfully'));
    return null;
  }
}