Fixed locales

Locales from Thunderbird instead of our own were used.
But it didn't work in Postbox.
Disabled attaching multiple files in open dialog

It doesn't work in Postbox at all.
In Thunderbird there is problem with labels of attachments in list.
We want nice labels (only filenames) but Lightning sets them to ugly
labels (full uris). Yes we can rewrite labels once Lightning has changed
them but it takes different times. And the time is the longer
the more attachments at once are selected. Causing the later attachments
to have ugly labels. No timeout can be used because of different time lengths.
Nor I was succesfull using some event listeners or MutationObserver.