Restrict Attachments by Mimetype on a Page Tree
Steps
Limit the file types that may be attached to one Page and every Page below it, using the "Attachments Restriction" section of the Page administration rather than the wiki-wide lists.
- Open the Page at the top of the tree you want to restrict, and select "Administer Page" in the "More Actions" menu.

- Select "Attachments Restriction", under "Content".

- Fill "Allowed Mimetypes" with the mimetypes you accept, or "Blocked Mimetypes" with the ones you refuse, separated by commas.
image/*,application/pdf - Select "Save".
- Attach a file of a refused type to a Page below the one you administered: the upload is turned down there too.

FAQ
Which Pages does the restriction cover?
The Page you administered and every Page below it, however deep the tree goes. A Page outside that tree keeps whatever the wiki sets.
Can a Page tree accept more than the rest of the wiki?
Yes. The lists you set replace the ones above instead of narrowing them, so a tree can take file types the wiki refuses everywhere else.
How do I lift the restriction from a Page tree?
Empty both fields and save. The tree then follows the wiki lists, and no longer the lists of any Page above it: a Page stops reading its ancestors as soon as it carries a restriction of its own, filled or not.
Do the two lists have to come from the same place?
No. Each field falls back on its own, so a tree may set "Blocked Mimetypes" and still take "Allowed Mimetypes" from the wiki.