As far as I can see I'm opening the same 2.8 mb file from the network share on mac and windows. Hidden files and underscore files are explicitly visible on windows for me 'coz I don't use any explorer-like interface, and different file types are colored differently in the file manager. For the experiment I even used FastStone image viewer, which opened the .heic just fine under Windows. The thing is, you can choose external programs to launch from the context menu ('Edit with external program') in FastStone viewer for the currently opened file. I have Affinity Photo and GIMP set there. So, if I open the .heic from cyrillic folder with the FSViewer, it opens fine. If I choose GIMP via 'Edit with external program', it opens fine. If I choose Affinity Photo from the same menu, I get the error.