taildrop: remove breaking abstraction layers for apple (#10728)

Removes the avoidFinalRename logic and all associated code as it is no longer required by the Apple clients.
Enables resume logic to be usable for Apple clients.

Fixes tailscale/corp#14772

Signed-off-by: Rhea Ghosh <rhea@tailscale.com>

taildrop/taildrop.go

@@ -90,15 +90,6 @@ type ManagerOptions struct {
 	// copy them out, and then delete them.
 	DirectFileMode bool
 
-	// AvoidFinalRename specifies whether in DirectFileMode
-	// we should avoid renaming "foo.jpg.partial" to "foo.jpg" after reception.
-	//
-	// TODO(joetsai,rhea): Delete this. This is currently depended upon
-	// in the Apple platforms since it violates the abstraction layer
-	// and directly assumes how taildrop represents partial files.
-	// Right now, file resumption does not work on Apple.
-	AvoidFinalRename bool
-
 	// SendFileNotify is called periodically while a file is actively
 	// receiving the contents for the file. There is a final call
 	// to the function when reception completes.
@@ -244,6 +235,7 @@ func (m *Manager) IncomingFiles() []ipn.PartialFile {
 			DeclaredSize: f.size,
 			Received:     f.copied,
 			PartialPath:  f.partialPath,
+			FinalPath:    f.finalPath,
 			Done:         f.done,
 		})
 		return true