add option to update PDF file of a problem in /problem/edit/:id
(grafted from 15b2554bf365f8639b5e94db4f1d9a277007808a)

--HG--
extra : source : 15b2554bf365f8639b5e94db4f1d9a277007808a
Merge pull request #9 from nattee/master

add option to update PDF file of a problem in /problem/edit/:id