Historically, photographers wanting to use filters did so by screwing one into a ring at the front of a lens. With no flipping mirror like a DSLR, and with extra open space in the FTZ Adapter, some new options have started to appear. Note that as I write and post this, these options are in various stages of pre-release (one is a Kickstarter project), and may not yet be available for purchase in the US.
Filter in Front of Sensor
The Sense Tech Innovation Corporation (STC) in Taiwan has a clip-in filter system that sits within the lens mount, just in front of the image sensor. The unique aspect of this is that you buy one filter that works with all your lenses (including ones that don't have filter rings). STC makes neutral density filters in 4, 6, 9, and 10 stop values, infrared filters at 590nm, 720mn, and 850nm values, UV-IR cut filters at 615nm, 625nm, 635nm, and 650nm, and an Astro filter that provides a narrow band response targeted for astrophotography.
Likewise, Kase has a set of clip-in filters for the Z5, Z6, Z6II, Z7, Z7II, Z9: ND8, ND32, ND64, ND1000, UV, and Neutral Night. These filters should eventually be available at B&H, this site's exclusive advertiser.
Kolari has clip in filters for the Z6 II, Z7 II, that also work with the Z5, Z6, Z7, and Z9 (magnetic clip is weaker in these latter cameras). These filters are available in an array of options: 3, 6, and 10 stop ND, mist diffusion (with or without ND), color enhancing, light pollution removing, various IR and UVIR-cut filters, and a clear quartz filter.
Astronomik has clip in filters for the Z5, Z6, Z6II, Z7, and Z7II bodies that provide a variety of astrophotography options (e.g. H-alpha).
Update: removed Aurora Aperture filter mention and link as they appear to have gone out of business.
