refactor import function

src/fileUtils.js

@@ -13,5 +13,31 @@ export default {
     // Clean up
     URL.revokeObjectURL(url)
     downloader.remove()
+  },
+
+  /**
+   * Note: if user press Cancel in file choosing dialog
+   * it will be an unsettled promise. But it's grabbed by
+   * the garbage collector (tested with FinalizationRegistry).
+   */
+  importFile () {
+    return new Promise((resolve, reject) => {
+      const uploader = document.createElement('input')
+
+      uploader.id = 'file-uploader'
+      uploader.type = 'file'
+      uploader.accept = '.json'
+
+      uploader.addEventListener('change', () => {
+        const file = uploader.files[0]
+        const reader = new FileReader()
+        reader.onload = (e) => {
+          resolve(e.target.result)
+        }
+        reader.readAsText(file)
+      })
+
+      uploader.click()
+    })
   }
 }