Make sure file sizes are localized

[pdehaan]

Subtle detail from #349 (comment):

It should be fully localized to allow reordering, something like

// This message is displayed when uploading a file, e.g. "1,3 MB of 10 MB".
uploadStatusMessage = { $size_loaded } of { $size_total }

Note how the comment says "1,3 MB". Currently we're using bytes to calculate file sizes for upload and download, but I'm not sure that it supports localizations.