Kar, there is no way you can know whether an editor supports saving back
to a given IEditorInput. If you check the API for the ISEI, it doesn't
even mention saving back to the underlying IStorage and suggests that a
new file resource be created for writing the contents.
"File-oriented editors should support this as a valid input type, and
display its content for viewing (but not allow modification). Within the
editor, the "save" and "save as" operations should create a new file
resource within the workspace."