Simplify showing/hiding attached file name

Note we have to render the `<p>` tag in one line because at the time of
writing browsers don't consider elements with whitespace inside as empty
[1].

[1] https://developer.mozilla.org/en-US/docs/Web/CSS/:empty

app/assets/javascripts/documentable.js

@@ -92,7 +92,6 @@
     },
     clearFilename: function(data) {
       $(data.fileNameContainer).text("");
-      $(data.fileNameContainer).hide();
     },
     clearInputErrors: function(data) {
       $(data.errorContainer).find("small.error").remove();
@@ -102,7 +101,6 @@
     },
     setFilename: function(data, file_name) {
       $(data.fileNameContainer).text(file_name);
-      $(data.fileNameContainer).show();
     },
     setProgressBar: function(data, klass) {
       $(data.progressBar).find(".loading-bar").addClass(klass);