What's the problem with capital I?
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.
As there is no standard to which Excel conforms as it predates standards and there would be an outcry if Excel started rejecting files that had worked for years.