made file downloading and caching smarter

tries eTag and ContentDisposition headers for file name+format
catches offline and server error/url error scenarios
creates a "recent" file copy if successful, and returns that as a fallback in an error scenario, if it exists

src/TED/TED.Utils/FileUtilities.cs

@@ -12,13 +12,19 @@ namespace TED.Utils
     internal class FileUtilities
     {
         /// <summary>
-        /// Downloads a file from a specified URL and saves it to a cache.
+        /// Downloads a file from a specified URL and saves it to a local cache.
         /// If the file already exists in the cache, the function returns the path to the cached file.
         /// If the file does not exist in the cache, the function downloads the file, saves it to the cache, and then returns the path to the cached file.
+        /// In the event of an error with file retrieval, the most recently used file will be returned, if it exists.
         /// </summary>
         /// <param name="url">The URL of the file to download.</param>
         /// <returns>A task that represents the asynchronous operation. The task result is the path to the downloaded (or cached) file.</returns>
         public static async Task<string> DownloadAndCacheFileAsync(string url)
+        {
+            var tedDirectory = Path.Combine(Path.GetTempPath(), "TED");
+            var recentPath = Path.Combine(tedDirectory, "recent.png");
+
+            try
             {
                 using (var client = new HttpClient()
                 {
@@ -26,22 +32,48 @@ namespace TED.Utils
                 })
                 {
                     var response = await client.GetAsync(url);
+
+                    try
+                    {
                         response.EnsureSuccessStatusCode();
-
-                var etag = response.Headers.ETag?.Tag.Replace("\"", string.Empty) ?? "untagged";
-                if (!Directory.Exists(Path.Combine(Path.GetTempPath(), "TED")))
-                {
-                    Directory.CreateDirectory(Path.Combine(Path.GetTempPath(), "TED"));
                     }
-                var path = Path.Combine(Path.GetTempPath(), "TED", $"{etag}.png");
+                    catch (HttpRequestException e)
-
-                if (File.Exists(path))
                     {
-                    return path;
+                        // If we catch here, it's a URL error or server-side issue.
+                        if (File.Exists(recentPath))
+                        {
+                            return recentPath;
                         }
 
-                var filesToDelete = Directory.GetFiles(Path.Combine(Path.GetTempPath(), "TED"), "*.png")
+                        return string.Empty;
-                                        .Where(filePath => Path.GetFileNameWithoutExtension(filePath) != etag);
+                    }
+
+                    var fileName = "ted.png";
+
+                    if (response.Headers.ETag != null)
+                    {
+                        fileName = $"{response.Headers.ETag?.Tag.Replace("\"", string.Empty)}.png";
+                    }
+                    else if (response.Content.Headers.ContentDisposition != null)
+                    {
+                        fileName = $"{response.Content.Headers.ContentDisposition.FileName}";
+                    }
+
+                    if (!Directory.Exists(tedDirectory))
+                    {
+                        Directory.CreateDirectory(tedDirectory);
+                    }
+
+                    var downloadPath = Path.Combine(tedDirectory, fileName);
+
+
+                    if (File.Exists(downloadPath))
+                    {
+                        return downloadPath;
+                    }
+
+                    var filesToDelete = Directory.GetFiles(tedDirectory)
+                                            .Where(filePath => Path.GetFileNameWithoutExtension(filePath) != fileName && Path.GetFileNameWithoutExtension(filePath) != "recent");
                     foreach (var fileToDelete in filesToDelete)
                     {
                         File.Delete(fileToDelete);
@@ -49,13 +81,28 @@ namespace TED.Utils
 
                     using (var stream = await response.Content.ReadAsStreamAsync())
                     {
-                    using (var fileStream = new FileStream(path, FileMode.CreateNew))
+                        using (var fileStream = new FileStream(downloadPath, FileMode.CreateNew))
                         {
                             await stream.CopyToAsync(fileStream);
-                        return path;
+                            if(File.Exists(recentPath))
+                            {
+                                File.Delete(recentPath);
+                            }
+                            File.Copy(downloadPath, recentPath);
+                            return downloadPath;
                         }
                     }
                 }
+            } catch(HttpRequestException e)
+            {
+                // If we catch here, we're offline.
+                if (File.Exists(recentPath))
+                {
+                    return recentPath;
+                }
+
+                return string.Empty;
+            }
         }
 
         /// <summary>