Filenames and file types | Google developer documentation style guide
Key Point: Use all-lowercase, and separate words with hyphens.Make file and directory names lowercase. In general, separate words with hyphens, not underscores. Use only standard ASCII alphanumeric characters in file and directory names. If you're adding to a directory where everything else already uses underscores, and it's not feasible to change everything to hyphens, it's okay to use underscores to stay consistent.
by Minkyoung Kim