This is required because the PDF Viewer itself is embedded using an iframe from the same domain. The default policy is blocking this.
Going on further, we have to come up with a solution in the future how to handle previews by applications, one example might be that they call their own endpoint and not the generic share page to allow applications to have full control over how to display previews.
Anyways, to test this behaviour use a decent newer browser (such as Chrome 41) and share a PDF file, obviously the PDF viewer needs to be enabled as well. Without this patch publicly shared PDF files should not get previewed and an error is thrown. (if it isn't then your browser is probably not obeying our Content-Security-Policy and you might consider switching to another one ;))
The "dir" key is used within the public sharing template to indicate in which directory the user currently is when sharing a directory with subdirectories. This is needed by the JS scripts.
However, when not accessing a directory then "dir" was set to the relative path of the file (from the user's home directory), meaning that for every public shared file the sharee can see the path.
(For example if you share the file "foo.txt" from "finances/topsecret/" the sharee would still see the path "finances/topsecret/" from the shared HTML template)
This is not the excpected behaviour and can be considered a privacy problem, this patch addresses this by setting "dir" to an empty key.
