Prevent XSS in RawHTML blocks by sanitizing output. (#4204)
Prevent XSS in RawHTML blocks by sanitizing output. (#4204)

* Prevent XSS in RawHTML blocks by sanitizing output.

Restrict allowed mimetypes for page document blocks.

* Make sure the test also checks if the file will be private

Co-authored-by: Ernst Odolphi <903d00b858ef41d3088a4ef9bd7c516f0ac5b1e7@onepercentclub.com>
Co-authored-by: Loek van Gent <7a5b6ccf5cffa35c5049b24807731ed7a4fdb4e7@goodup.com>
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