Theme-Dev: Options-Page > File conversion after upload

Hi there,

I’m developing my first WP-Theme right now and I need an Options-Page for the following Usecase:
– User uploads CSV which gets converted to JSON and the path saved to the Themes Options.

I created a Theme Options-Page from that tutorial:
https://code.tutsplus.com/articles/how-to-integrate-the-wordpress-media-uploader-in-theme-and-plugin-options–wp-26052

I then changed it to use the new uploader according to that tutorial:
https://www.webmaster-source.com/2013/02/06/using-the-wordpress-3-5-media-uploader-in-your-plugin-or-theme/

Ok, the upload works! ??

What I want to do now is: After the upload of the CSV, convert it immediately to JSON (have the code for that) and save the path of the new file to the Options.
Where should I do that and how?