Have a look at the junior jobs: GitHub · Where software is built
The task itself will need you to understand the file list code from the files app:core/apps/files/js/filelist.js at master · owncloud/core · GitHub and also parts of the gallery app GitHub - owncloud/gallery: 🌅 Gallery app for ownCloud, which includes previews for all supported media files that need to be adjusted to be a “thumbnails” view replacement for the file list.