Update save dialog suffix when the format filter changes

ShowSaveFileDialog set the default suffix once from the configured
extension and never listened to QFileDialog::filterSelected. Picking
another image format in the file type dropdown therefore had no effect
on the saved file: a name typed without an extension still got the
configured suffix, and with a native dialog such as GTK's the extension
already present in the name was not swapped either, so it had to be
edited by hand.

Connect filterSelected, derive the suffix from the selected mime type
and make it the new default suffix. If the current name already ends in
one of the offered image formats, rename it to the new suffix as well;
the Qt widget dialog does that on its own, native dialogs do not.

Fixes #4708

Author: 4RH1T3CT0R7

src/utils/screenshotsaver.cpp

@@ -104,6 +104,34 @@ QString ShowSaveFileDialog(const QString& title, const QString& directory)
       QMimeDatabase().mimeTypeForFile("image." + suffix).name();
     dialog.selectMimeTypeFilter(defaultMimeType);
     dialog.setDefaultSuffix(suffix);
+    // Follow the format picked in the file type dropdown: use its suffix
+    // when the typed name has none, and swap the extension of a name that
+    // already has one (the Qt dialog does this itself, GTK's does not).
+    QObject::connect(&dialog,
+                     &QFileDialog::filterSelected,
+                     &dialog,
+                     [&dialog](const QString&) {
+                         QString newSuffix =
+                           QMimeDatabase()
+                             .mimeTypeForName(dialog.selectedMimeTypeFilter())
+                             .preferredSuffix();
+                         if (newSuffix.isEmpty()) {
+                             return;
+                         }
+                         dialog.setDefaultSuffix(newSuffix);
+                         QStringList files = dialog.selectedFiles();
+                         if (files.isEmpty()) {
+                             return;
+                         }
+                         QFileInfo info(files.constFirst());
+                         QString oldSuffix = info.suffix().toLower();
+                         if (oldSuffix != newSuffix &&
+                             QImageWriter::supportedImageFormats().contains(
+                               oldSuffix.toLatin1())) {
+                             dialog.selectFile(info.dir().filePath(
+                               info.completeBaseName() + "." + newSuffix));
+                         }
+                     });
     if (dialog.exec() == QDialog::Accepted) {
         return dialog.selectedFiles().constFirst();
     } else {