Rework of the file export (#894)

Co-authored-by: Thorsten Sommer <SommerEngineering@users.noreply.github.com>

app/MindWork AI Studio/Assistants/BatchProcessing/BatchProcessingOutputMode.cs

@@ -6,9 +6,14 @@ namespace AIStudio.Assistants.BatchProcessing;
 public enum BatchProcessingOutputMode
 {
     /// <summary>
-    /// One Markdown result file per processed document.
+    /// One result file per processed document, written in the chosen file format.
     /// </summary>
-    MARKDOWN_FILES,
+    /// <remarks>
+    /// This must stay the first member. Enums are persisted under their name, and an unknown name
+    /// falls back to the default value of the enum, which is the member with the value zero. That
+    /// is what lets settings written before this member was renamed still land here.
+    /// </remarks>
+    INDIVIDUAL_FILES,
 
     /// <summary>
     /// A CSV results table, where each AI answer becomes one row. The content of