Fix links for TNEF attachements with umlauts.

This patch fixes the generated file links for files in TNEF attachments containing
non-ascii-charcters. With this patch, the same file naming scheme is applied as
used in KNefParser::extractAttachementTo(...) which first uses att->fileName()
and att->file() only as a fallback. The patch contains fixes for all code snippets
in KDEPIM using KTNEFParser::extractFileTo(...).

REVIEW: 121937