When creating a file uploader for your website, it will be quite handy if users may see a preview of their uploaded content prior to using their precious bandwidth to send over the file. This is what Promosis, a project created by John Datserakis, aims to accomplish using CSS. It is a simple piece of source code that acts as a front end to give web users or webmasters a visualization of their upload progress. A live demo is even provided on his Github page.
Giving the source code a try is completely free thanks to the project's MIT License. You may also modify to fit the needs of your website or even to expand the list of supported files. The style of this open source tool is based on Bootstrap 4's custom file-upload. The installation is really easy and it only requires the basic ability to use NPM from the command line. It may have to kinks to work out for mobile developers, but the library is extensive enough to work with.