Basically, Excel uses the equivalent of ‘file’ (https://man7.org/linux/man-pages/man1/file.1.html), sees the magic “ID”, and decides a SYLK file, even though .csv files starting with “ID” have outnumbered .SYLK files by millions for decades.
Thanks. So I guess the easy compatible solution is to always quote the first item on the first line when writing CSV. Good to know. (Checking if the item starts with ID is more work. Possibly quote all items on the first line for simplicity.) (Reading SYLK is obviously irrelevant, so accepting unquoted ID when reading is the smarter way to go and will actually improve compatibility with writers that are not Excel. Also it takes no work.)
