There are 2 problems here, 1. export portfolio files not displaying utf8 correctly, and 2. export portfolio utf8 filename not being presented by IE correctly.
1. is fixed due to image rewriter method not reading/encoding file in utf8 when it rewrites image links;
2. is not a LAMS problem due to browser support of character encodings in the content-disposition header. This page http://greenbytes.de/tech/tc2231/
has a good roundup of browser behaviour in this regard. Note that IE and Safari usually don't try to convert to utf8 and neither support RFC2231 (specifying encoding in header).