Yes, that looks OK, though you don't need the With .. End With structure given you're only doing one thing with the range:
Code:
Private Sub txtEnclosureAttached_Update()
Dim Rng As Range
Set Rng = ActiveDocument.Bookmarks(bmkEnclosureAttached).Range
Rng.Text = Replace(Enclosures.Text, vbCr, ", ")
Application.ScreenRefresh
ActiveDocument.Bookmarks.Add bmkEnclosureAttached, Rng
End Sub
You also don't need the 'Application.ScreenRefresh' unless it's important to be able to see the document update.
PS: Please use code tags when posting code - you can find them on the Advanced tab.