Unknown file types include .txt files and any other file type that Alteryx cannot read natively. If you use an Input Data Tool to read an unknown file type, the Resolve File Type window displays with the following options:
Read it as a built in type: Select the file type that resembles the structure of your file.
Read it as a fixed width text file: When you select this option, you can assign columns by using a sliding ruler. See Flat File Layout for more information on this option.
Read it as a delimited text file: Select this option if there is a single delimiter that signifies column breaks in your data. Select an appropriate delimiter type, and optionally select whether the First Row Contains Field Names in your data.