In reality, is almost certainly a piece of alternate reality gaming (ARG) or a "creepypasta." The Alphanumeric naming convention (a letter followed by four digits) mimics the default naming settings of certain older digital cameras, which lends the story a sense of mundane authenticity.
Legend says that once the file is downloaded, it "mirrors" itself, appearing in different folders on the user's computer under different names, making it nearly impossible to delete. Fact vs. Fiction
By using such a boring, technical filename, creators tap into our fear of the "glitch in the machine"—the idea that something malevolent could be hiding in a standard system file. Why Do We Obsess Over It?