LPS-36716 Wiki portlet throws incorrect error when invalid file extension is uploaded

In reference to the questions on https://github.com/matthewkong/liferay-portal/pull/258:

The JSON object is used to follow the convention implemented in LPS-29543 for EditFileEntryAction and then again in LPS-34707 in ImportLayoutsAction. I asked Marc why he originally implemented it and this was his response:

"Hmmm. I believe I had to handle the 'uploadcomplete' event asynchonously in JS by binding it to JS functionality:

uploader.on('uploadcomplete', instance._onUploadComplete, instance);

I couldn't think of a way for JSP to handle the user upload event for me."

Then, in regards to removing the error tags in edit_page_attachment.jsp, those errors messages are used for the fall back uploader so we can't remove them.

If you have any questions, please feel free to let me know.