Tertius

I had this thumbnail issue as well. At some point all my *.pdn files showed the default paint.net icon instead of a thumbnail, and I never found the cause. Today I got to the bottom of this. It's Onedrive files on demand. Thumbnail handling outside of a onedrive folder always works perfectly. Thumbnail handling within a OneDrive synced folder appears broken. It is connected with Onedrive Files on demand. If you activate files on demand (Onedrive settings->Files on demand), Windows explorer downloads thumbnails from the Onedrive cloud service instead of creating them from local files, even if the files are stored locally. The cloud service doesn't seem to have a thumbnail handler for *.pdn files, so no thumbnail is downloaded and the generic icon is displayed. Solution: deactivate Onedrive Files on demand in the Onedrive Settings app. Thumbnail downloading from the cloud is immediately replaced with local thumbnail generating. You see the difference immediately after you uncheck this setting. It's not enough to keep the setting active and mark *.pdn files to be stored locally ("always keep on this device" in explorer). Instead, it's necessary to actually uncheck this setting.