FIX: Replace %20 with space in markdown file name for uploads (#8405)

Meta: https://meta.discourse.org/t/image-name-has-20-in-file-name/134136

We were ending up with [file%20name](url) in the markdown preview, which looked weird and
affected the alt text. this is because we were calling encodeURIComponent, which has been left in place because this is a valid thing to do for some cases. (e.g. f674b9e)

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

@@ -200,6 +200,10 @@ QUnit.test("getUploadMarkdown", assert => {
     testUploadMarkdown("[foo|bar].png"),
     "![%5Bfoo%7Cbar%5D|100x200](/uploads/123/abcdef.ext)"
   );
+  assert.equal(
+    testUploadMarkdown("file name with space.png"),
+    "![file name with space|100x200](/uploads/123/abcdef.ext)"
+  );
 
   const short_url = "uploads://asdaasd.ext";