A folder should get a folder mimetype

If doing achunked upload the mimetype of the folder would otherwise be
guessed from the path. Which always returned application/octet-stream.

If an access control rule to block that is in place this means that all
chunked uploads fail hard in directories as the isCreatable on the
directory always fails.

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>