Resources on handling file uploads.

http://php.net/manual/en/features.file-upload.php
http://php.net/manual/en/reserved.variables.post.php#87512
https://www.sitepoint.com/file-uploads-with-php/
https://www.sitepoint.com/monitoring-file-integrity/

Front end

https://www.quirksmode.org/dom/inputfile.html

https://www.sitepoint.com/tracking-upload-progress-with-php-and-javascript/

http://stackoverflow.com/questions/29727436/upload-progress-bar-using-session-in-php-5-4-with-codeigniter

Related

https://www.sitepoint.com/monitoring-file-integrity/

https://www.sitepoint.com/html5-native-drag-and-drop-api/

Sulphur

https://github.com/KorvinM/sulphur