Saving the content of dcc.upload()

Hi all
I am trying to understand the application of https://dash.plotly.com/dash-core-components/upload

In my case the user uploads an image and i am writting a callback that takes the contents of that upload and save it in the server

Currently I am getting the “contents” property of the upload image, but there is an error that is a list… is it something with decoding first?