Fix react imports. (#2589)

packages/@uppy/react/index.mjs

@@ -1,5 +1,5 @@
-export { default as Dashboard } from './lib/Dashboard'
-export { default as DashboardModal } from './lib/DashboardModal'
-export { default as DragDrop } from './lib/DragDrop'
-export { default as ProgressBar } from './lib/ProgressBar'
-export { default as StatusBar } from './lib/StatusBar'
+export { default as Dashboard } from './lib/Dashboard.js'
+export { default as DashboardModal } from './lib/DashboardModal.js'
+export { default as DragDrop } from './lib/DragDrop.js'
+export { default as ProgressBar } from './lib/ProgressBar.js'
+export { default as StatusBar } from './lib/StatusBar.js'