The FujiFilm X-T2 mirrorless camera has a native ISO range from 200 to 12800.
There are also a couple of extended options--100 on the bottom end and 25600 and 51200 on the top end. These extended ones are quite serviceable, but you do take a hit in dynamic range and overall image quality. But they might also mean getting the shot or not, so it's wherever you want to weight the compromise.
FujiFilm X-T2 ISO Sample Images
Here's a collection of images across the range of available ISO values on the X-T2. These were all shot in aperture priority mode on a tripod with manual ISO settings. They were shot directly to JPG using the Provia/Standard film emulation, the default DR100 dynamic range setting, auto white balance, Adobe RGB colorspace, and all the image quality settings (such as noise reduction, sharpness, color, and shadow tone) zeroed out. There hasn't been any post-processing applied, and you download the original JPGs as they were straight out of the camera.
To give the best idea of what the camera does, I deliberately haven't done anything to improve the images or mitigate the effects of the high ISO settings. But that's something you can tackle in software like Lightroom or any of the other image processing apps, particularly if you shoot in RAW.
And with these, I deliberately shot something with smooth tonal gradations from light to dark to show up any issues with noise (or mottling, as the Fujifilm owner's manual puts it). It's also a good subject to test the sensor's dynamic range.
With the small versions embedded below it's hard to see any real differences (that, in itself, is a testament to how good its high ISO performance is), so I've included links to the original straight-out-of-camera versions (click on each image to open its corresponding original).
Auto ISO Settings on the Fujifilm X-T2
When using the Auto ISO setting, you have a few options for customizing how it behaves.
You can set the default sensitivity. The default is the lowest native ISO (200), and in many situations, that's going to be ideal in that it starts at the lowest native ISO and only goes higher if it needs to. But there are also situations where you might want to make sure it's using a high ISO.
There are three presets (Auto 1, Auto 2, and Auto 3) that you can set to maximum sensitivity limits. So, for instance, if you're shooting portraits and don't want it to go over, say, ISO 800, you could set that as the maximum for Auto 1 and use Auto 1 when shooting portraits. But if you want to switch to shooting run and gun low-light street scenes, you might want to set Auto 2 to something like ISO 6400.
You can also designate the minimum shutter speed. Some other cameras can set this from the lens focal length, but on the X-T2 you don't have that option though can set an overall minimum shutter speed. The default is 1/60 sec. One thing to note, though, is that this isn't a hard floor. That is, the camera will override it if the image is still underexposed even when using the maximum ISO that you've set in the Max Sensitivity setting.