Text files have txt extensions. If you’d care to be more precise it would help us to help you.
As so often, the Online Help would have told you that by entering this question in the Search field: http://help.adobe.com/en_US/indesign/cs/using/WSa285fff53dea4f8617383751001ea8cb3f-6f41a.h tml
But here is the full list: Word .doc, .docx; RTF; plain text (in a variety of encodings).
Thanks for the help Jongware that was what I needed to know