The Concrete5 Legacy multi file uploader requires flash which is no longer enabled by default in most browsers. This is a set of core file overrides which will replace the flash based uploader with Dropzone allowing for multi file uploads without the flash browser requirement.
To use, copy the files into their respective places under the root c5 directory, except the readme file, then clear the cache and the multi-file, flash-based uploader in the File Manager should be replaced with Dropzone.