On clicking 'edit' button in grid view, it should auto populate the FileUploadControl textbox with the path name (of the image stored in db of type varbinary'

I tried the following statement, but it throws error.

FileUploadControl.PostedFile.FileName = (byte[])dsModuleDetails.Tables[0].Rows[0]["InvoiceFile"];

By "throws error" do you mean "does not compile"? Because HttpPostedFile.FileName is a read-only property. Which makes sense, as there hasn't been any file posted yet. Also, it's a string, not a byte array.

Have a look through the FileUpload control documentation; you might find something else you can use. I don't know this control very well, but the FileName property is read-only, too. I don't see anything else that might set the filename.