Added macOS FilePicker implementation

Samples/Samples/ViewModel/FilePickerViewModel.cs

@@ -77,6 +77,7 @@ async void DoPickCustomType()
                     { DevicePlatform.Android, new[] { "application/comics" } },
                     { DevicePlatform.UWP, new[] { ".cbr", ".cbz" } },
                     { DevicePlatform.Tizen, new[] { "*/*" } },
+                    { DevicePlatform.macOS, new[] { "cbr", "cbz" } }, // or general UTType values
                 });
 
             var options = new PickOptions

Xamarin.Essentials/FilePicker/FilePicker.macos.cs

@@ -0,0 +1,105 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Threading.Tasks;
+using AppKit;
+using MobileCoreServices;
+
+namespace Xamarin.Essentials
+{
+    public static partial class FilePicker
+    {
+        static Task<FilePickerResult> PlatformPickFileAsync(PickOptions options)
+        {
+            var openPanel = new NSOpenPanel
+            {
+                CanChooseFiles = true,
+                AllowsMultipleSelection = false,
+                CanChooseDirectories = false
+            };
+
+            if (options.PickerTitle != null)
+                openPanel.Title = options.PickerTitle;
+
+            SetFileTypes(options, openPanel);
+
+            FilePickerResult result = null;
+            var panelResult = openPanel.RunModal();
+            if (panelResult == (nint)(long)NSModalResponse.OK)
+            {
+                result = new FilePickerResult(openPanel.Urls[0].Path);
+            }
+
+            return Task.FromResult(result);
+        }
+
+        static Task<IEnumerable<FilePickerResult>> PlatformPickMultipleFilesAsync(PickOptions options)
+        {
+            var openPanel = new NSOpenPanel
+            {
+                CanChooseFiles = true,
+                AllowsMultipleSelection = true,
+                CanChooseDirectories = false
+            };
+
+            if (options.PickerTitle != null)
+                openPanel.Title = options.PickerTitle;
+
+            SetFileTypes(options, openPanel);
+
+            var resultList = new List<FilePickerResult>();
+            var panelResult = openPanel.RunModal();
+            if (panelResult == (nint)(long)NSModalResponse.OK)
+            {
+                foreach (var url in openPanel.Urls)
+                {
+                    resultList.Add(new FilePickerResult(url.Path));
+                }
+            }
+
+            return Task.FromResult<IEnumerable<FilePickerResult>>(resultList);
+        }
+
+        static void SetFileTypes(PickOptions options, NSOpenPanel panel)
+        {
+            var allowedFileTypes = new List<string>();
+
+            if (options?.FileTypes?.Value != null)
+            {
+                foreach (var type in options.FileTypes.Value)
+                {
+                    allowedFileTypes.Add(type.TrimStart('*', '.'));
+                }
+            }
+
+            panel.AllowedFileTypes = allowedFileTypes.ToArray();
+        }
+    }
+
+    public partial class FilePickerFileType
+    {
+        public static FilePickerFileType PlatformImageFileType() =>
+            new FilePickerFileType(new Dictionary<DevicePlatform, IEnumerable<string>>
+            {
+                { DevicePlatform.macOS, new string[] { UTType.PNG, UTType.JPEG, "jpeg" } }
+            });
+
+        public static FilePickerFileType PlatformPngFileType() =>
+            new FilePickerFileType(new Dictionary<DevicePlatform, IEnumerable<string>>
+            {
+                { DevicePlatform.macOS, new string[] { UTType.PNG } }
+            });
+    }
+
+    public partial class FilePickerResult
+    {
+        internal FilePickerResult(string filePath)
+            : base(filePath)
+        {
+            FileName = Path.GetFileName(filePath);
+        }
+
+        Task<Stream> PlatformOpenReadStreamAsync()
+            => Task.FromResult<Stream>(File.OpenRead(FullPath));
+    }
+}