Fixed. It turned out I wasn't following the spec (
http://www.w3.org/Protocols/rfc2616/rfc2616-sec19.html) correctly. I didn't put quotes around the filename in the Content-Disposition.
As for the detection of various artists, you might be interested to know that there is some "fuzzy" logic applied. Sometimes (in my music collection, at least), there are different spellings, typos, case and punctuation for the same artist - for instance "R.E.M", "REM", "r.e.m" etc. I don't want to flag this as VA, so I use something called the Levenshtein distance (
http://en.wikipedia.org/wiki/Levenshtein_distance) to determine whether the artist names are fairly resembling. If the L distance between two artists in a directory is greater than 4, it is flagged as VA.
Do let me know if you find any negative side effects of this approach.
Sindre