Delay check for mimetype and plugin until required

If a thumbnail already exists, the mimetype of the original file and
available plugins do not matter.

Saves accessing the original file, and allows thumbnails for file types
which are not natively supported by KIO.

thumbnail/thumbnail.cpp

@@ -667,18 +667,16 @@ const QImage ThumbnailProtocol::getIcon()
 bool ThumbnailProtocol::createSubThumbnail(QImage& thumbnail, const QString& filePath,
                                            int segmentWidth, int segmentHeight)
 {
-    const QMimeDatabase db;
     const QUrl fileUrl = QUrl::fromLocalFile(filePath);
-    const QString subPlugin = pluginForMimeType(db.mimeTypeForUrl(fileUrl).name());
-    if (subPlugin.isEmpty() || !m_enabledPlugins.contains(subPlugin)) {
-        return false;
-    }
 
-    ThumbCreator* subCreator = getThumbCreator(subPlugin);
-    if (!subCreator) {
-        // qDebug() << "found no creator for" << dir.filePath();
-        return false;
-    }
+    auto getSubCreator = [&fileUrl, this]() -> ThumbCreator* {
+        const QMimeDatabase db;
+        const QString subPlugin = pluginForMimeType(db.mimeTypeForUrl(fileUrl).name());
+        if (subPlugin.isEmpty() || !m_enabledPlugins.contains(subPlugin)) {
+            return nullptr;
+        }
+        return getThumbCreator(subPlugin);
+    };
 
     if ((segmentWidth <= 256) && (segmentHeight <= 256)) {
         // check whether a cached version of the file is available for
@@ -710,7 +708,8 @@ bool ThumbnailProtocol::createSubThumbnail(QImage& thumbnail, const QString& fil
 
             QSaveFile thumbnailfile(thumbPath.absoluteFilePath(thumbName));
             bool savedCorrectly = false;
-            if (subCreator->create(filePath, cacheSize, cacheSize, thumbnail)) {
+            ThumbCreator* subCreator = getSubCreator();
+            if (subCreator && subCreator->create(filePath, cacheSize, cacheSize, thumbnail)) {
                 scaleDownImage(thumbnail, cacheSize, cacheSize);
 
                 // The thumbnail has been created successfully. Store the thumbnail
@@ -726,8 +725,9 @@ bool ThumbnailProtocol::createSubThumbnail(QImage& thumbnail, const QString& fil
                 thumbnailfile.commit();
             }
         }
-    } else if (!subCreator->create(filePath, segmentWidth, segmentHeight, thumbnail)) {
-        return false;
+    } else {
+        ThumbCreator* subCreator = getSubCreator();
+        return subCreator && subCreator->create(filePath, segmentWidth, segmentHeight, thumbnail);
     }
     return true;
 }