Update UploadModal to toggle thumbnail browse button based on mode

- Modified `js/ui/components/UploadModal.js` to hide the thumbnail browse button when in 'External Link' mode.
- Ensured the browse button is visible when switching back to 'Upload File' mode.
- Added logic to clear the thumbnail file input when resetting the form.

js/ui/components/UploadModal.js

@@ -258,9 +258,17 @@ export class UploadModal {
         if (!this.hasValidR2Settings() && this.sourceSections.settings.classList.contains("hidden")) {
             this.toggles.storageSettings.click(); // Expand
         }
+
+        if (this.toggles.browseThumbnail) {
+            this.toggles.browseThumbnail.classList.remove("hidden");
+        }
     } else {
         this.sourceSections.upload.classList.add("hidden");
         this.sourceSections.external.classList.remove("hidden");
+
+        if (this.toggles.browseThumbnail) {
+            this.toggles.browseThumbnail.classList.add("hidden");
+        }
     }
 
     // Update Button Text