implemented Nextcloud file previews for unknown file types using fileId and has-preview flags, updated file models, and refined manual refresh logic.

lib/widget/file_viewer.dart

@@ -3,6 +3,7 @@ import 'dart:convert';
 import 'dart:io';
 import 'dart:math';
 
+import 'package:cached_network_image/cached_network_image.dart';
 import 'package:chewie/chewie.dart';
 import 'package:file_picker/file_picker.dart';
 import 'package:flutter/material.dart';
@@ -14,6 +15,8 @@ import 'package:share_plus/share_plus.dart';
 import 'package:syncfusion_flutter_pdfviewer/pdfviewer.dart';
 import 'package:video_player/video_player.dart';
 
+import '../model/account_data.dart';
+import '../model/endpoint_data.dart';
 import '../routing/app_routes.dart';
 import '../share_intent/remote_file_ref.dart';
 import '../state/app/modules/settings/bloc/settings_cubit.dart';
@@ -22,6 +25,21 @@ import 'centered_leading.dart';
 import 'info_dialog.dart';
 import 'share_position_origin.dart';
 
+/// Nextcloud's `/index.php/core/preview` endpoint — returns a rasterized
+/// thumbnail for any file the server has a preview provider for (images,
+/// PDFs with the right backend, Office in some setups). Falls back to an
+/// HTTP 404 when no preview is available, which lets [CachedNetworkImage]
+/// trigger its `errorWidget`. Prefers `fileId` because the path variant
+/// is unreliable on some server configurations.
+String _ncPreviewUrl(RemoteFileRef remote, {int width = 1024}) {
+  final host = EndpointData().nextcloud().full();
+  final id = remote.fileId;
+  final selector = id != null
+      ? 'fileId=$id'
+      : 'file=${Uri.encodeQueryComponent(remote.path)}';
+  return 'https://$host/index.php/core/preview?$selector&x=$width&y=-1&a=1';
+}
+
 class FileViewer extends StatefulWidget {
   final String path;
   final bool openExternal;
@@ -482,52 +500,58 @@ class _FileViewerState extends State<FileViewer> {
     ),
   );
 
-  Widget _buildUnknownView() {
+  Widget _buildUnknownView() => Scaffold(
+    appBar: _appbar(showActionsMenu: false),
+    body: _buildUnknownPlaceholder(),
+  );
+
+  Widget _buildUnknownPlaceholder() {
     final theme = Theme.of(context);
     final descriptors = _availableActions();
-    return Scaffold(
-      appBar: _appbar(showActionsMenu: false),
-      body: ListView(
-        padding: const EdgeInsets.symmetric(vertical: 24),
-        children: [
-          Padding(
-            padding: const EdgeInsets.symmetric(horizontal: 24),
-            child: Column(
-              children: [
-                const Icon(Icons.insert_drive_file_outlined, size: 60),
-                const SizedBox(height: 16),
+    final remote = widget.remoteFile;
+    final hasPreview = remote?.hasPreview == true;
+    return ListView(
+      padding: const EdgeInsets.symmetric(vertical: 24),
+      children: [
+        Padding(
+          padding: const EdgeInsets.symmetric(horizontal: 24),
+          child: Column(
+            children: [
+              _UnknownPreviewHeader(remoteFile: remote),
+              const SizedBox(height: 16),
+              if (!hasPreview) ...[
                 Text(
                   'Vorschau nicht verfügbar',
                   style: theme.textTheme.titleMedium,
                   textAlign: TextAlign.center,
                 ),
                 const SizedBox(height: 6),
-                Text(
-                  widget.path.split('/').last,
-                  style: theme.textTheme.titleSmall?.copyWith(
-                    fontWeight: FontWeight.w600,
-                  ),
-                  textAlign: TextAlign.center,
-                ),
-                const SizedBox(height: 8),
-                Text(
-                  'Wähle eine Aktion, um mit der Datei weiterzuarbeiten.',
-                  style: theme.textTheme.bodySmall,
-                  textAlign: TextAlign.center,
-                ),
               ],
-            ),
+              Text(
+                widget.path.split('/').last,
+                style: theme.textTheme.titleSmall?.copyWith(
+                  fontWeight: FontWeight.w600,
+                ),
+                textAlign: TextAlign.center,
+              ),
+              const SizedBox(height: 8),
+              Text(
+                'Wähle eine Aktion, um mit der Datei weiterzuarbeiten.',
+                style: theme.textTheme.bodySmall,
+                textAlign: TextAlign.center,
+              ),
+            ],
           ),
-          const SizedBox(height: 24),
-          ...descriptors.map(
-            (d) => ListTile(
-              leading: CenteredLeading(Icon(d.icon)),
-              title: Text(d.label),
-              onTap: () => _handleAction(d.action),
-            ),
+        ),
+        const SizedBox(height: 24),
+        ...descriptors.map(
+          (d) => ListTile(
+            leading: CenteredLeading(Icon(d.icon)),
+            title: Text(d.label),
+            onTap: () => _handleAction(d.action),
           ),
-        ],
-      ),
+        ),
+      ],
     );
   }
 
@@ -583,6 +607,50 @@ class _TextPayload {
   const _TextPayload({required this.content, required this.truncated});
 }
 
+/// Header for the "Vorschau nicht verfügbar" screen: tries to fetch the
+/// Nextcloud thumbnail and shows it mid-sized if available, otherwise
+/// falls back to the generic file icon.
+class _UnknownPreviewHeader extends StatelessWidget {
+  final RemoteFileRef? remoteFile;
+  const _UnknownPreviewHeader({required this.remoteFile});
+
+  static const double _previewSize = 180;
+
+  Widget _fallbackIcon() =>
+      const Icon(Icons.insert_drive_file_outlined, size: 60);
+
+  @override
+  Widget build(BuildContext context) {
+    final remote = remoteFile;
+    // Skip the probe outright when the server already told us there is no
+    // preview — saves an HTTP request that would 404 anyway.
+    if (remote == null || remote.hasPreview == false) return _fallbackIcon();
+    return SizedBox(
+      width: _previewSize,
+      height: _previewSize,
+      child: CachedNetworkImage(
+        httpHeaders: AccountData().authHeaders(),
+        imageUrl: _ncPreviewUrl(remote, width: 360),
+        fadeInDuration: Duration.zero,
+        fadeOutDuration: Duration.zero,
+        errorListener: (_) {},
+        placeholder: (_, _) =>
+            const Center(child: AppProgressIndicator.large()),
+        errorWidget: (_, _, _) => Center(child: _fallbackIcon()),
+        imageBuilder: (_, imageProvider) => ClipRRect(
+          borderRadius: BorderRadius.circular(8),
+          child: Image(
+            image: imageProvider,
+            fit: BoxFit.contain,
+            width: _previewSize,
+            height: _previewSize,
+          ),
+        ),
+      ),
+    );
+  }
+}
+
 class _MediaPlayer extends StatefulWidget {
   final String path;
   final bool isAudio;