What is the correct capitalization for "zip file? "
Solution 1:
Standard usage (dating back to its original 1993 MIME registration) is "ZIP file." Even though it's not an acronym, the use of all caps denotes that you are talking about a file in a specific format that is not to be confused with the normal English word "zip." That said, I would also be fine with "zip file" as you propose, which comes off as a little less ostentatious.
However, it is not a proper noun and I would avoid "ZIP File" (or "Zip File") entirely.