The short answer is, the Cloudflare® Content Delivery Network (CDN) caches web assets based on its file type. Only certain file types are cached and only files that do not exceed 512MB are cached. Please see the provided list of current file types that are automatically cached:
bmp, ejs, jpeg, pdf, ps, ttf, class, eot, jpg, pict, svg, webp, css, eps, js, pls, svgz, woff, csv, gif, mid, png, swf, woff2, doc, ico, midi, ppt, tif, xls, docx, jar, otf, pptx, tiff, and xlsx.
For a more in-depth answer to this question, please refer to the Cloudflare article "Understanding the Cloudflare CDN".