Use {{image-uploader}} tag in plugin , and I hope use image in server-side

I want to use {{image-uploader}} in my plugin.

like in profile.hbs
{{image-uploader imageUrl=model.card_background_upload_url type="card_background"}}

I don’t know how to get the image set for this image-uploder.
<form {{ action 'testAction' evidence on='submit' }}>
and tried to get it in testAction, but I could not figure out how to get it when using image-uploader.
I tried to follow the process by referring to user.hbs, but I couldn’t figure out which action I was transitioning to.

I want to process images in ruby, how can I pass images to server side using image-uploader?

I would appreciate it if you could tell me.