One can disable the automatic upload by not setting the uploadOnChange property to true, and then adding a change handler to the control. The change handler has access to the uploaded files via oEvent.getParameters().files, and can process the uploaded files. When the form that is inside the FileUploader control is never submitted, it won't try to upload the files.

<mvc:View
    controllerName="app.controller.Main"
    xmlns="sap.m" xmlns:mvc="sap.ui.core.mvc" xmlns:unified="sap.ui.unified"
    displayBlock="true">
    <Shell id="shell">
        <App id="app">
            <pages>
                <Page id="page" title="{i18n>title}">
                    <content>
                        <unified:FileUploader id="ui5-uploader"/>
                    </content>
                </Page>
            </pages>
        </App>
    </Shell>
</mvc:View>

sap.ui.define([
    "sap/ui/core/mvc/Controller"
], function (Controller) {
    "use strict";

    return Controller.extend("app.controller.Main", {
        onAfterRendering: function() {
            var uploader = this.getView().byId("ui5-uploader");
            uploader.attachChange(function(oEvent) {
                oEvent.getParameters().files[0].text().then(
                    function(sContent) {
                        console.log(sContent);
                    }
                );
            });
        }
        
    });
});