I typically use the following settings with regard to file tabs:
File tab sort order: Manual
New file tab position: New files on right
These settings are honored with regard to the 'legacy' File Tabs (toolbar) but not for the new-style v18 document tabs. New-style files/tabs seem to always appear 'on the left' no matter what settings I try.
This looks especially weird if you happen to open the File Tabs toolbar (not that you may want to anymore). With the settings above, the actual document tabs will be in 'mirrored order' as compared with those in the File Tabs toolbar.
Win7 x64