Keep `file:///` for absolute paths in HTML exports

Closes #1155

DiscordChatExporter.Core/Exporting/ExportContext.cs

@@ -124,9 +124,9 @@ internal class ExportContext
             var filePath = await _assetDownloader.DownloadAsync(url, cancellationToken);
             var relativeFilePath = Path.GetRelativePath(Request.OutputDirPath, filePath);
 
-            // Prefer relative paths so that the output files can be copied around without breaking references.
-            // If the asset directory is outside of the export directory, use an absolute path instead.
-            var optimalFilePath =
+            // Prefer the relative path so that the export package can be copied around without breaking references.
+            // However, if the assets directory lies outside of the export directory, use the absolute path instead.
+            var shouldUseAbsoluteFilePath =
                 relativeFilePath.StartsWith(
                     ".." + Path.DirectorySeparatorChar,
                     StringComparison.Ordinal
@@ -134,15 +134,19 @@ internal class ExportContext
                 || relativeFilePath.StartsWith(
                     ".." + Path.AltDirectorySeparatorChar,
                     StringComparison.Ordinal
-                )
-                    ? filePath
-                    : relativeFilePath;
+                );
+
+            var optimalFilePath = shouldUseAbsoluteFilePath ? filePath : relativeFilePath;
 
             // For HTML, the path needs to be properly formatted
             if (Request.Format is ExportFormat.HtmlDark or ExportFormat.HtmlLight)
             {
-                // Create a 'file:///' URI and then strip the 'file:///' prefix to allow for relative paths
-                return new Uri(new Uri("file:///"), optimalFilePath).ToString()[8..];
+                // Format the path into a valid file URI
+                var href = new Uri(new Uri("file:///"), optimalFilePath).ToString();
+
+                // File schema does not support relative paths, so strip it if that's the case
+                // https://github.com/Tyrrrz/DiscordChatExporter/issues/1155
+                return shouldUseAbsoluteFilePath ? href : href[8..];
             }
 
             return optimalFilePath;