On pc Debian x86-64 with master sources updated yesterday, multiselection doesn't work, it only retain 1 file.
There's a inprogress rework for multiselection (see http://nabble.documentfoundation.org/Multiselection-needs-work-tt4153207.html). First "getFiles" method which has different implementations according to the env (Win, MacOs, Kde, gtk,...) has been deprecated. getSelectedFiles should be used now since all implementation behave the same way, retrieve the list of files with their full path.
Julien Nabet committed a patch related to this issue. It has been pushed to "master": http://cgit.freedesktop.org/libreoffice/core/commit/?id=af814158b101418adad4f9a21d3b2303620b0396 tdf#94351: Web Wizard multiselection doesn't work It will be available in 5.1.0. The patch should be included in the daily builds available at http://dev-builds.libreoffice.org/daily/ in the next 24-48 hours. More information about daily builds can be found at: http://wiki.documentfoundation.org/Testing_Daily_Builds Affected users are encouraged to test the fix and report feedback.
There has been several changes about multiselection (for example: prefer getSelectedFiles method over getFiles). So this one will be available only from 5.1.0