How to send and then receive node js files?

Question

Node.js

There is an application on the server (linux), it converts the image into different formats, works on NODE JS, the essence of the application is to put the file in the folder with the application, execute the command, take the files from the folder.
How can this be linked to the site? Those. uploaded the file, waited for the conversion process to finish, downloaded the file.
Tell me the algorithm of action, I know how to upload a file, but how to send a command for execution, how to understand that the process is over? How is the interaction with server applications through the web interface generally built?
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