A user can make a shortcut/alias to a file

Test Pre-requisites:
The user has private files.

Test Steps:
1. Edit your private files; launch file picker, pick a file from (file system)/(dropbox)/(server files) repo plugin respectively, and check/tick "Create an alias/shortcut to the file"
2. Click added file in file tree and make sure it is viewable
3. Create a file or folder resource, pick this file from private area as reference, save it and make sure this file is visible
4. Create a file or folder resource, pick the same file from private area as a copy, save it and make sure this file is visible
5. Update the file in the external repository and confirm that the file is also updated in #3 but is not updated in #4

(Note that the cache needs a purge for the thumbnail of a linked image to change over.)