There is a wizard you can use to import txt files. You will be given some options as to which character separates the fields. That´s when you choose tab.
If you click the advanced tab, you will be able to save your import settings (your importspecification), not only with the tab delimited but with field types etc.
So what I meant was (I realize I forgot to mention that) that you do one import manually to save an importspecification (with tab delimited).