There's no preview because we feel the internal file browser does the job nicely, just as it did in FT2. Click Disk Op. and the tiny button marked FLIP to access it. You just have to remember to select an unused instrument slot before you load an instrument in order not to overwrite your existing ones. When you actually load a sample/instrument this way, you can try it out in any pitch. "Tracker samples" often come in varying quality and one set sample rate for previewing wouldn't work all that well for a wide lo-fi - hi-fi range.
Protip: While you have to double click files in the internal browser to load them, the right mouse button only needs one click.