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

This allows us to upload only the PDF file to the available task without uploading all the testdata file.

(grafted from 15b2554bf365f8639b5e94db4f1d9a277007808a)

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extra : source : 15b2554bf365f8639b5e94db4f1d9a277007808a