Improve Syncing Obsidian Vault, Invalidate Static Assets in Browser Cache in Web Client (#657)

- Improve
  - Only send files modified since their last sync for indexing on server from the Obsidian client
- Fix 
  - Invalidate static asset browser cache in Web client when Khoj version changes

src/interface/desktop/main.js

@@ -106,6 +106,7 @@ function filenameToMimeType (filename) {
         case 'org':
             return 'text/org';
         default:
+            console.warn(`Unknown file type: ${extension}. Defaulting to text/plain.`);
             return 'text/plain';
     }
 }