Running Camera roll from unity app

In an app we're currently working on porting to IOS, we want to save a screenshot and then allow the user to immediately view the saved screenshot.

In Android/Windows this works by simply using Application.openURL(...), yet in IOS this doesn't appear to be doing anything

(we're using a 3rd party plugin that saves the image and then runs a callback with the saved file path)

This is the code that functions as expected on the Windows version of the Unity editor and Android (opening the file in photo viewer/gallery once the function is called):

 public void openFile()
 {
     if (Application.platform == RuntimePlatform.Android) //open the file on android
     {
         string tgt = fileToOpen.Trim('/');
         Debug.Log("Opening " + tgt);
         Application.OpenURL(tgt);
     }
     else //open the file in the editor (only functions in windows)
     {

         string tgt = "file://" + fileToOpen.Trim('/');
         Debug.Log("Opening " + tgt);
         Application.OpenURL(tgt);
     }
 }

FileToOpen is constructed like this:

 Application.PersistentDataPath+"/"+albumname+"/"+filename+".png"

We've also noticed that IOS claims the fileToOpen path is:

 /Users/[username]/Library/Application Support/[company name]/[project name]/[albumname]/[filename].png

Yet the file is saved to

 /Users/[username]/[project name]/[albumname]/[filename].png

(Note that /Users/[username]/[project name] is the source code path for the project, and the paths are correct on Windows and Android)

TL;DR: I save a .png file to the Camera Roll in iOS. Then when I ask it to open this .png file, nothing happens. I want to open the Camera Roll and view this file, not in my app but in the iOS Camera Roll app.