Allow reliable uploads of large files

There's a need for attaching large files (for instance confluence export packages in the Confluence Migrator Pro application).

A failure after 75% of a 15G upload is frustrating. On a somewhat unreliable network, not having a robust upload mechanism can make it impossible to upload what needs to be attached.

When an error occurs, XWiki should retry the segment that failed and go on. Another way to allow uploads to go through would be to allow users to upload segments of files, for instance using split zip archives.