If you’re using Safari, that’s probably the culprit. Safari will try to actually open .rar files under 100MB right in the browser window, rather than downloading them fully. That doesn’t work, because Safari can’t read the .rar format. The file needs to be downloaded entirely before opening, using a .rar-compatible compression utility.
Generally, the solution is to use “option-click” to properly download the file, rather than just clicking on the link. Otherwise, we recommend using another browser, such as Opera, Chrome, or Firefox, since those programs automatically download small .rar files like these normally.