Your operating system is guessing what type of file it is based on the .art file extension, which it doesn't recognize, but it really is a text file, even if it doesn't have the .txt extension that your OS expects. You can generally force your text editor to open it anyway.
I thought Thonny would open any file, but one student on a Mac had an issue where Thonny wouldn't let them change the filter to "all files", so they opened the .ART file with TextEdit instead.