Essentials/Xamarin.Essentials/FilePicker/FilePicker.shared.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Threading.Tasks;

namespace Xamarin.Essentials
{
    public static partial class FilePicker
    {
        public static async Task<FileResult> PickAsync(PickOptions options = null) =>
            (await PlatformPickAsync(options))?.FirstOrDefault();

        public static Task<IEnumerable<FileResult>> PickMultipleAsync(PickOptions options = null) =>
            PlatformPickAsync(options ?? PickOptions.Default, true);
    }

    public partial class FilePickerFileType
    {
        public static readonly FilePickerFileType Images = PlatformImageFileType();
        public static readonly FilePickerFileType Png = PlatformPngFileType();
        public static readonly FilePickerFileType Jpeg = PlatformJpegFileType();
        public static readonly FilePickerFileType Videos = PlatformVideoFileType();
        public static readonly FilePickerFileType Pdf = PlatformPdfFileType();

        readonly IDictionary<DevicePlatform, IEnumerable<string>> fileTypes;

        protected FilePickerFileType()
        {
        }

        public FilePickerFileType(IDictionary<DevicePlatform, IEnumerable<string>> fileTypes) =>
            this.fileTypes = fileTypes;

        public IEnumerable<string> Value => GetPlatformFileType(DeviceInfo.Platform);

        protected virtual IEnumerable<string> GetPlatformFileType(DevicePlatform platform)
        {
            if (fileTypes.TryGetValue(platform, out var type))
                return type;

            throw new PlatformNotSupportedException("This platform does not support this file type.");
        }
    }

    public class PickOptions
    {
        public static PickOptions Default =>
            new PickOptions
            {
                FileTypes = null,
            };

        public static PickOptions Images =>
            new PickOptions
            {
                FileTypes = FilePickerFileType.Images
            };

        public string PickerTitle { get; set; }

        public FilePickerFileType FileTypes { get; set; }
    }
}