After Openfire's part of HTTPFileUpload plugin has been updated to 1.1.0 version, it stopped working in Spark. It still works with 1.0.0 version on the server. Spark's part needs to be updated to be compatible with 1.1.0 version of that plugin. Not sure about backwards compatibility as 2.9.0 of Spark hasn't been released and is not widely used yet.
dnsdhrj's patch fixed the compatibility issue