FIX: be more tactical with replacing markdown chars

* instead of using encodeURIComponent in imageNameFromFileName,
  we just replace the bad characters that we wanted to get rid
  of in the first place where we introduced encodeURIComponent.
  as per review

test/javascripts/lib/uploads-test.js.es6

@@ -198,7 +198,7 @@ QUnit.test("getUploadMarkdown", assert => {
   );
   assert.equal(
     testUploadMarkdown("[foo|bar].png"),
-    "![%5Bfoo%7Cbar%5D|100x200](/uploads/123/abcdef.ext)"
+    "![foobar|100x200](/uploads/123/abcdef.ext)"
   );
   assert.equal(
     testUploadMarkdown("file name with space.png"),