Add QFileIconProvider implementation using mime database

If built-in QFileIconProvider does not seem to work, use custom
implementation which queries mime database.

src/gui/torrentcontentmodel.cpp

@@ -37,6 +37,9 @@
 #include <Windows.h>
 #include <Shellapi.h>
 #include <QtWin>
+#else
+#include <QMimeDatabase>
+#include <QMimeType>
 #endif
 
 #include "guiiconprovider.h"
@@ -85,6 +88,49 @@ namespace
             return QIcon(iconPixmap);
         }
     };
+#else
+    /**
+     * @brief Tests whether QFileIconProvider actually works
+     *
+     * Some QPA plugins do not implement QPlatformTheme::fileIcon(), and
+     * QFileIconProvider::icon() returns empty icons as the result. Here we ask it for
+     * two icons for probably absent files and when both icons are null, we assume that
+     * the current QPA plugin does not implement QPlatformTheme::fileIcon().
+     */
+    bool doesQFileIconProviderWork()
+    {
+        QFileIconProvider provider;
+        const char PSEUDO_UNIQUE_FILE_NAME[] = "/tmp/qBittorrent-test-QFileIconProvider-845eb448-7ad5-4cdb-b764-b3f322a266a9";
+        QIcon testIcon1 = provider.icon(QFileInfo(
+            QLatin1String(PSEUDO_UNIQUE_FILE_NAME) + QLatin1String(".pdf")));
+        QIcon testIcon2 = provider.icon(QFileInfo(
+            QLatin1String(PSEUDO_UNIQUE_FILE_NAME) + QLatin1String(".png")));
+
+        return (!testIcon1.isNull() || !testIcon2.isNull());
+    }
+
+    class MimeFileIconProvider: public UnifiedFileIconProvider
+    {
+        using QFileIconProvider::icon;
+        QIcon icon(const QFileInfo &info) const override
+        {
+            const QMimeType mimeType = m_db.mimeTypeForFile(info, QMimeDatabase::MatchExtension);
+            QIcon res = QIcon::fromTheme(mimeType.iconName());
+            if (!res.isNull()) {
+                return res;
+            }
+
+            res = QIcon::fromTheme(mimeType.genericIconName());
+            if (!res.isNull()) {
+                return res;
+            }
+
+            return UnifiedFileIconProvider::icon(info);
+        }
+
+    private:
+        QMimeDatabase m_db;
+    };
 #endif
 }
 
@@ -95,7 +141,8 @@ TorrentContentModel::TorrentContentModel(QObject *parent)
 #ifdef Q_OS_WIN
     m_fileIconProvider = new WinShellFileIconProvider();
 #else
-    m_fileIconProvider = new QFileIconProvider();
+    static bool doesBuiltInProviderWork = doesQFileIconProviderWork();
+    m_fileIconProvider = doesBuiltInProviderWork ? new QFileIconProvider() : new MimeFileIconProvider();
 #endif
 }