Currently we only have for uploading files, this will help resolve MOBILE-2256

How this works:
Tthere is a WS to prepare a draft area file and then a WS to save the draft area files to the user private files (replacing its existing contents with the new draft area)

This is consistent with how LMS works, the app will:
1. Prepare the draft area once the user selects to edit their private files (core_user_prepare_private_files_for_edition)
2. Use an existing WS to remove elements from existing draft areas
3. Save the draft area into the users private files (replacing its current contents) with core_user_update_private_files

Is the same approach used in similar WS such as mod_glossary_prepare_entry_for_edition or core_blog_prepare_entry_for_edition