FileDownloadWindow.cs

using System;
using System.ComponentModel;
using System.IO;
using System.Net;
using UnityEditor;
using UnityEngine;

public sealed class FileDownloadWindow : EditorWindow
{
    private string              m_address       ;
    private UnityEngine.Object  m_folder        ;
    private bool                m_isDownloading ;

    [MenuItem( "Window/File Download Window" )]
    private static void Init()
    {
        var win = GetWindow<FileDownloadWindow>();
        win.minSize = win.maxSize = new Vector2( 320, 80 );
    }

    private void OnGUI()
    {
        GUI.enabled = !m_isDownloading;

        EditorGUIUtility.labelWidth = 50;

        m_address = EditorGUILayout.TextField( "URL", m_address );

        EditorGUI.BeginChangeCheck();

        var folder = EditorGUILayout.ObjectField( "Folder", m_folder, typeof( UnityEngine.Object ), false );

        if ( EditorGUI.EndChangeCheck() )
        {
            if ( folder != null )
            {
                var path = AssetDatabase.GetAssetPath( folder );
                var attr = File.GetAttributes( path );

                if ( ( attr & FileAttributes.Directory ) != 0 )
                {
                    m_folder = folder;
                }
            }
            else
            {
                m_folder = null;
            }
        }

        EditorGUIUtility.labelWidth = 0;

        GUI.enabled = !m_isDownloading && !string.IsNullOrEmpty( m_address ) && m_folder != null;

        if ( GUILayout.Button( "Download" ) )
        {
            Download();
        }

        GUI.enabled = true;
    }

    private void Download()
    {
        var wc          = new WebClient();
        var address     = new Uri( m_address );
        var filename    = Path.GetFileName( m_address );
        var directory   = AssetDatabase.GetAssetPath( m_folder );
        var path        = directory + "/" + filename;

        wc.DownloadProgressChanged  += OnChanged;
        wc.DownloadFileCompleted    += OnCompleted;

        wc.DownloadFileAsync( address, path );

        m_isDownloading = true;
    }

    private void OnChanged( object sender, DownloadProgressChangedEventArgs e )
    {
        var progress    = e.ProgressPercentage / 100f;
        var cancel      = !EditorUtility.DisplayCancelableProgressBar
        (
            title       : "FileDownloadWindow",
            info        : m_address,
            progress    : progress
        );

        if ( !cancel ) return;

        var wc = sender as WebClient;
        wc.CancelAsync();
    }

    private void OnCompleted( object sender, AsyncCompletedEventArgs e )
    {
        EditorUtility.ClearProgressBar();
        AssetDatabase.Refresh();

        m_isDownloading = false;
    }
}