Fix #2773 (#2841)

* Added an API endpoint to check the existence of the file

* fix #2773: Now we can prevent users from posting the same images

* bug fix

src/client/app/common/scripts/get-md5.ts

@@ -0,0 +1,8 @@
+const crypto = require('crypto');
+
+export default (data: ArrayBuffer) => {
+  const buf = new Buffer(data);
+  const hash = crypto.createHash("md5");
+  hash.update(buf);
+  return hash.digest("hex");
+};