I'm working on a program that is writing a UTF-8 text file. This process works fine. What doesn't work fine is that TextEdit.app (and others) does not recognise it automatically as a UTF-8 text ...

If the files are utf-8, the filesystem isn't going to care. Whatever application you use to look at the files just needs to interpret them as utf-8. More options ...

In this build, Microsoft added the ability to save files as UTF-8 without a BOM (Byte Order Mark), which is labeled as the "UTF-8" option when saving a file. New UTF-8 without BOM option.

Choose "UTF-8" from the drop-down box next to "Encoding" and click "Save." Your text file will be converted and saved in the UTF-8 format, although the file extension will remain the same.