If I understand you correctly, you'll need to use the Event API to create an observer for the assignsubmission_file\event\assessable_uploaded event. This could technically be any type of plugin, it depends how if at all you want it to integrate into Moodle's interface. If it's completely separate to Moodle's existing workflows and just operates in the background when a submission is made, it could be a Local plugin.
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Develop new plugin for assignments level
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