I am looking for the correct term to use when writing about a .zip file (presumably the answer will be the same for other file types as well). I am working on the documentation for a project, and both ".zip" and "zip" is used by other authors before. Sources on the internet seem to be inconsistent and I can't find any straight forward answer on the matter. Any real sources on which one is the correct term is very welcome.
The most trusted source I find is the Python documentation, which seems to consistently use "zip". Link.
Meanwhile, the Wikipedia page seems to mix "zip" and ".zip". Link.
zip is accepted as the name of the lossless compression format developed in the late 1980s by Phil Katz.
.zip denotes a file extension that is probably associated with that format. Not all zip formats have the .zip extension; notable examples are the .jar files of Java, and the newer Excel formats .xlsx and .xlsm.
If I were you, I'd adopt the former (i.e. zip), and amend Wikipedia if you get the chance.
(Cf. tarball and .tar).