This tutorial https://docs.magnolia-cms.com/product-docs/6.2/Developing/Development-how-tos/How-to-add-an-asset-with-REST.html doesn't work completely because you need to be sure that when an asset is uploaded the size of that asset needs to be written to the jcr:content subnode.
Without the size the asset cannot be used correctly.
Follow the instructions to upload the asset (2 rest calls). Then open the asset in the detail subapp or try to use the asset or try to download the asset. Doesn't work.
Not sure how to fix it. Somehow the size of the asset needs to be sent on the second request. Here is how we do it in java when the assets is uploaded through the UI.