View Document in Attachment Object using Visualforce and Apex Class

Apex Class :

//insertAttachment

public with sharing class insertAttachment {
    public blob attachVal {get; set;}
    public string attachName {get; set;}
    public Id AttachId;

    public insertAttachment() {
        AttachId = ApexPages.CurrentPage().getParameters().get(‘Id’);
    }

    public PageReference insertNewFile() {
        try {
            delete [select Id from attachment where ParentId =: AttachId ];
            Blob b = attachVal;
            Attachment at = new Attachment(Name=attachName, body=b, ParentId=AttachId);
            insert at;
        } catch(Exception e){ }
        return null;
    }

    public List getAttachmentList() {
        List attachList = new List();
       if(AttachId != null) {
            string attachQry = ‘Select Id, Name, Body, ParentId from Attachment where ParentId =: AttachId’;
            attachList = (List) Database.query(attachQry);
        }
        return attachList;
    }
}

VisualForce Page: