On Linux and the BSDs, where the save dialog reports the file type
picked in it, a name that still ends in the suggested name's extension
gets the picked type's extension. The comparison was case-sensitive,
so Untitled-1.VECTORCRAFT with "PDF-compatible .ai" picked was saved
as a native file named Untitled-1.VECTORCRAFT instead of Untitled-1.ai.
with_picked_type now compares the lower-case extensions that
fileio::extension gives. A name typed with the picked type's own
extension in another case (a.AI with .ai picked) stays as typed.
This follows up #3372.
Docs: docs/target-app-parity.md.