Special Characters in Attachment fail to upload (e.g. Umlaute)

Hello,

after upgrading from phpbb3.0.12 to 3.1.2 I noticed that uploading of files with special characters other than _ a-z and 0-9 are failing during the upload.

phpbb doesn't seem to convert those special characters correctly and breaks the name with using ?? instead of the character.
This problem comes with php3.1.x when posting and writing PMs and doesn't seem to be a Server-/Client side problem, since it works with php3.0.x on the same setup.

Is it a bug or a feature?

Best regards