Last updated: Dec 30, 2016.
These are registry files designed to add thumbnails to Windows XP, Vista, 7, 8 and 10 for stereo MPO files, as produced by Fuji's 3D cameras. This will essentially tell Windows that these files are the same as JPEG files, which is mostly true. An MPO file is two JPEG images stored in sequence in a single file, so just about any program that can open a JPEG file can open an MPO, if you can get it to ignore the extension. All Microsoft programs I've tried can open an MPO just fine, because Microsoft programs check the header of the file rather than the extension when trying to determine the type of file. So the standard Windows thumbnail generator and property reader can read thumbnails and properties from MPO files, because it thinks they are JPEG files. The thumbnails will only show the left eye view, but that's probably what you'd want anyway.
Even after applying this registry change, you will still be able to set a different default handler for MPO files from the one for JPEG files. For example, you could set JPEGs to be opened by PhotoShop, and MPOs to be opened by StereoPhoto Maker.
Important note! If you use Windows property editing features to change a property on an MPO, it will resave the file, and since it thinks the file is a jpeg, it will only save the first image in the file, making the file no longer a stereo file. Obviously this is very bad. The same problem, it should be noted, will happen with any program that can read JPEG files but is not aware of MPO; it's fine to use these programs to read MPOs for previewing purposes, but if they save the images for any reason, it will destroy the second image.
You may want to make your original MPO images read-only, which will prevent any program from saving over them. In Windows 7, 8 and 10 (and probably Vista), you can select all the MPOs, right-click one of them and select "Properties". Then click the "read-only" checkbox. This is a good idea in any case; if you want to realign or edit the files you can do it on a copy, as a side-by-side JPEG.
This first file is designed to clear out any current settings, including default handler, for the MPO extension. Right-click the link and select "Save target".
It's not a bad idea to start by applying this file, though it does mean that you'll need to tell Windows (again) to open MPOs with StereoPhoto Maker or nVidia Photo Viewer or whatever you are used to using. You don't need to apply it first, but I prefer to start from scratch. If you prefer, you can apply one of the files below and it should add the thumbnails without changing your default handler. But if you get odd behavior, or you just want to reset everything back to as it was when the computer was new, download this file, then double-click it. The computer will warn you about applying registry settings; click yes. Then reboot and the computer will be completely unaware of MPO files; just like it was when you first bought it.
This has three versions, one for XP and two for Vista and Windows 7: the first with a full set of properties in the preview pane, and the second with all of the editable properties removed, as an extra safeguard against accidentally editing the MPO properties. Right-click the appropriate link and select "Save target".
MPO thumbnails registry file for Windows XP
MPO thumbnails registry file for Windows 7, 8, 10 and Vista
Once you download the correct file and save it somewhere useful (like the desktop), double-click the file to add the settings to the registry. The computer will put up a warning about applying registry files. Say "yes". Then reboot. If you now go to a folder containing MPO files, you should see both a thumbnail and setting when you click on the file. You can also change the view to "thumbnails" and see the whole folder as thumbnail images.
If you apply the file, but don't see thumbnails when you go to a folder with MPOs in it, you may need to clear the thumbnail cache, so Windows will generate new thumbnails for files you've already looked at. Some links to tutorials about how to do that are linked below, or just search for "clear thumbnail cache windows 10" or substitute Windows 7 or whatever you're using.
Clearing the thumbnail cache in Windows 10
Clearing the thumbnail cache in Windows 7
For some reason, IE seems to change the extension of .REG files to .HTM. If you download the individual files, you may need to rename them from (something).htm to (something).reg.
Alternatively, here's a ZIP file with all three registry files in it:
On XP, the slideshow feature doesn't work, and the "filmstrip" view option for folders doesn't work. Apparently there is some "previewer" object that needs to be associated with MPOs in a way I don't yet understand.
Comments and questions: email me.