How to upload an attachment using the API?

[Dkf]

I'm writing a PHP script for uploading images using an API for a plugin (not my own).
I noticed that the uploaded images in this plugin have the following parameters in the database:

content_type = "plugin name"
content_id = "plugin-specific id"

How do I pass these details via the API?
How to upload an attachment using the API?

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