Sanitize file names when exporting media

Fixes #332

DiscordChatExporter.Domain/Exporting/ExportRequest.cs

@@ -2,6 +2,7 @@
 using System.IO;
 using System.Text;
 using DiscordChatExporter.Domain.Discord.Models;
+using DiscordChatExporter.Domain.Internal;
 
 namespace DiscordChatExporter.Domain.Exporting
 {
@@ -127,8 +128,7 @@ namespace DiscordChatExporter.Domain.Exporting
             buffer.Append($".{format.GetFileExtension()}");
 
             // Replace invalid chars
-            foreach (var invalidChar in Path.GetInvalidFileNameChars())
-                buffer.Replace(invalidChar, '_');
+            PathEx.EscapePath(buffer);
 
             return buffer.ToString();
         }

DiscordChatExporter.Domain/Exporting/MediaDownloader.cs

@@ -27,10 +27,11 @@ namespace DiscordChatExporter.Domain.Exporting
         {
             var originalFileName = Regex.Match(url, @".+/([^?]*)").Groups[1].Value;
 
-            if (string.IsNullOrWhiteSpace(originalFileName))
-                return GetRandomSuffix();
+            var fileName = !string.IsNullOrWhiteSpace(originalFileName) ?
+                $"{Path.GetFileNameWithoutExtension(originalFileName)}-{GetRandomSuffix()}{Path.GetExtension(originalFileName)}" :
+                GetRandomSuffix();
 
-            return $"{Path.GetFileNameWithoutExtension(originalFileName)}-{GetRandomSuffix()}{Path.GetExtension(originalFileName)}";
+            return PathEx.EscapePath(fileName);
         }
 
         // HACK: ConfigureAwait() is crucial here to enable sync-over-async in HtmlMessageWriter

DiscordChatExporter.Domain/Internal/PathEx.cs

@@ -0,0 +1,18 @@
+using System.IO;
+using System.Text;
+
+namespace DiscordChatExporter.Domain.Internal
+{
+    internal static class PathEx
+    {
+        public static StringBuilder EscapePath(StringBuilder pathBuffer)
+        {
+            foreach (var invalidChar in Path.GetInvalidFileNameChars())
+                pathBuffer.Replace(invalidChar, '_');
+
+            return pathBuffer;
+        }
+
+        public static string EscapePath(string path) => EscapePath(new StringBuilder(path)).ToString();
+    }
+}