I think you have misunderstood how the FileUploader works.
The "uploadUrl" parameter should be used to specify a path on the "web server" (e.g. application server, web container) on which your application is hosted. UI5 is a web user interface framework, it does not know how to handle (server-side) upload requests. This means that the server (backend) itself should have some implementation for handing the file upload.
After you select the file and trigger the upload, a POST HTTP request is made to the path specified in this "uploadUrl" parameter. If you have no web server to know to handle it, then it will invariably give back an error HTTP response.
Based on the title of your question, I understand that you would want to upload the file inside your workspace. IMO, this does not really make sense (as you are mixing in the design time environment with your run-time environment (i.e. your application should never depend on the IDE).
Nevertheless, you can try and import a file via the import menu (right click on package, import, from file system) and see what URL is the request triggered against (using the dev console). I looked around a little and roughly this is the request URL: http://localhost:[Web IDE Port]/xfer/import/[User Name]-OrionContent/[Project Name]
. In the Slug
header you would have the file name. You might not be able to make a POST request towards this URL directly (because of XSS / CSS limitations), so you might need to create a route mapping for it.