Hi Praveen,
What there is no OOB feature for this,for achieving this what you can do is:
1)Create one Custom Dialog
2)In the jsp have one output Text component whose value will be populated from the backend bean of dialog.
3)then in init of Dialog Class you can get the currentNodeRef read the content and populate the property mentioned in step 2 with the contents outputStream.
Refer wiki for Custom Dialog in Alfresco.