org-publish-sitemap-sort-folders

org-publish-sitemap-sort-folders is a variable defined in `ox-publish.el'.
Its value is

first

Documentation:
A symbol, denoting if folders are sorted first in sitemaps.
Possible values are `first', `last', and nil.
If `first', folders will be sorted before files.
If `last', folders are sorted to the end after the files.
Any other value will not mix files and folders.

You can overwrite this default per project in your
`org-publish-project-alist', using `:sitemap-sort-folders'.

You can customize this variable.