Fix type error in FileUploader (#1280)

Summary:
Introduced in #1277.

Test Plan:
Run `yarn start` and visit <http://localhost:8080/test/FileUploader/>.
Conduct the test plan as specified on that page.

wchargin-branch: fileuploader-target

src/util/FileUploader.js

@@ -24,8 +24,11 @@ export class FileUploader extends React.Component<Props> {
     const onUpload = (e) => {
       const file = e.target.files[0];
       const reader = new FileReader();
-      reader.onload = (e) => {
-        const jsonString = e.target.result;
+      reader.onload = () => {
+        const jsonString = reader.result;
+        if (typeof jsonString !== "string") {
+          throw new Error("Unexpected: jsonString is not string");
+        }
         const json = JSON.parse(jsonString);
         this.props.onUpload(json);
       };