Return a correct filter from QFileDialog::selectedMimeTypeFilter()

QFileDialog::selectedMimeTypeFilter() returns either an empty
filter in the case when a platform file dialog doesn't implement
mime type filters, or initiallySelectedMimeTypeFilter() in the
case of Qt's file dialog. In both cases the result is incorrect.

Make it return a mime type filter corresponding to a selected
name filter. As a result, tst_QFiledialog::setMimeTypeFilters()
has to be fixed: QFileDialog::selectMimeTypeFilter() can't select
a name filter for an invalid mime type, and "application/json"
is not supported by RHEL 6.6, so replace it by "application/pdf".

Change-Id: I58d3be860a9b5e8a72cba86d74b520178115a812
Reviewed-by: David Faure <david.faure@kdab.com>

src/widgets/dialogs/qfiledialog.cpp

@@ -1462,19 +1462,6 @@ void QFileDialog::selectNameFilter(const QString &filter)
     }
 }
 
-/*!
- * \since 5.9
- * \return The mimetype of the file that the user selected in the file dialog.
- */
-QString QFileDialog::selectedMimeTypeFilter() const
-{
-    Q_D(const QFileDialog);
-    if (!d->usingWidgets())
-        return d->selectedMimeTypeFilter_sys();
-
-    return d->options->initiallySelectedMimeTypeFilter();
-}
-
 /*!
     \since 4.4
 
@@ -1611,6 +1598,36 @@ void QFileDialog::selectMimeTypeFilter(const QString &filter)
 
 #endif // QT_NO_MIMETYPE
 
+/*!
+ * \since 5.9
+ * \return The mimetype of the file that the user selected in the file dialog.
+ */
+QString QFileDialog::selectedMimeTypeFilter() const
+{
+    Q_D(const QFileDialog);
+    QString mimeTypeFilter;
+    if (!d->usingWidgets())
+        mimeTypeFilter = d->selectedMimeTypeFilter_sys();
+
+#ifndef QT_NO_MIMETYPE
+    if (mimeTypeFilter.isNull() && !d->options->mimeTypeFilters().isEmpty()) {
+        const auto nameFilter = selectedNameFilter();
+        const auto mimeTypes = d->options->mimeTypeFilters();
+        for (const auto &mimeType: mimeTypes) {
+            QString filter = nameFilterForMime(mimeType);
+            if (testOption(HideNameFilterDetails))
+                filter = qt_strip_filters({ filter }).first();
+            if (filter == nameFilter) {
+                mimeTypeFilter = mimeType;
+                break;
+            }
+        }
+    }
+#endif
+
+    return mimeTypeFilter;
+}
+
 /*!
     \property QFileDialog::viewMode
     \brief the way files and directories are displayed in the dialog

tests/auto/widgets/dialogs/qfiledialog/tst_qfiledialog.cpp

@@ -1049,12 +1049,12 @@ void tst_QFiledialog::setMimeTypeFilters_data()
     QTest::addColumn<QString>("expectedSelectedMimeTypeFilter");
 
     const auto headerMime = QStringLiteral("text/x-chdr");
-    const auto jsonMime = QStringLiteral("application/json");
+    const auto pdfMime = QStringLiteral("application/pdf");
     const auto zipMime = QStringLiteral("application/zip");
 
     QTest::newRow("single mime filter (C header file)") << QStringList {headerMime} << headerMime << headerMime;
-    QTest::newRow("single mime filter (JSON file)") << QStringList {jsonMime} << jsonMime << jsonMime;
-    QTest::newRow("multiple mime filters") << QStringList {jsonMime, zipMime} << jsonMime << jsonMime;
+    QTest::newRow("single mime filter (JSON file)") << QStringList {pdfMime} << pdfMime << pdfMime;
+    QTest::newRow("multiple mime filters") << QStringList {pdfMime, zipMime} << pdfMime << pdfMime;
 }
 
 void tst_QFiledialog::setMimeTypeFilters()
@@ -1068,6 +1068,10 @@ void tst_QFiledialog::setMimeTypeFilters()
     fd.selectMimeTypeFilter(targetMimeTypeFilter);
 
     QCOMPARE(fd.selectedMimeTypeFilter(), expectedSelectedMimeTypeFilter);
+
+    auto *filters = fd.findChild<QComboBox*>("fileTypeCombo");
+    filters->setCurrentIndex(filters->count() - 1);
+    QCOMPARE(fd.selectedMimeTypeFilter(), mimeTypeFilters.last());
 }
 
 void tst_QFiledialog::setEmptyNameFilter()