Mixing Client-/Server-Side Tests

I wouldn't mix these two types of tests at all. You have several different concerns here. You can still get full test coverage without having to combine tests like you are describing.

• Client side unit tests (using Karma, Mocha, etc.) validate that the JavaScript code is behaving as expected.
• Server side unit tests (using JUnit or whatever unit testing framework exists for your back end) validate that the server code is behaving as expected.
• Integration or end-to-end tests (using Selenium) validate generally validate that multiple components are working together correctly. You can use a mock back-end for these if necessary.

You can create Karma/Mocha unit tests to verify the details of whatever processing may exist for that file upload action. Here, you should be testing edge cases of the individual JavaScript functions underneath the UI.

You can then create a Selenium test to purely test the upload action and expected result of uploading the file. You can verify that the correct elements exist on the page and have the expected properties after the upload. The JavaScript is a black box in this case. You are testing from the user's perspective. This will exercise some of the code you unit tested already, but the point of it is to test the connection between front and back ends.