Reviewed uses of type alias FileExtension, it belongs in more places

src/import/Import.cpp

@@ -350,7 +350,7 @@ bool Importer::Import(const FilePath &fName,
    AudacityProject *pProj = GetActiveProject();
    auto cleanup = valueRestorer( pProj->mbBusyImporting, true );
 
-   wxString extension = fName.AfterLast(wxT('.'));
+   const FileExtension extension{ fName.AfterLast(wxT('.')) };
 
    // Always refuse to import MIDI, even though the FFmpeg plugin pretends to know how (but makes very bad renderings)
 #ifdef USE_MIDI

src/import/ImportGStreamer.cpp

@@ -345,7 +345,7 @@ GStreamerImportPlugin::GetSupportedExtensions()
    // We refresh the extensions each time this is called in case the
    // user had installed additional gstreamer plugins while Audacity
    // was active.
-   mExtensions.Empty();
+   mExtensions.clear();
 
    // Gather extensions from all factories that support audio
    {

src/import/ImportPlugin.h

@@ -173,7 +173,7 @@ public:
       return mFormatName;
    }
 
-   bool SupportsExtension(const wxString &extension)
+   bool SupportsExtension(const FileExtension &extension)
    {
       return mExtensions.Index(extension, false) != wxNOT_FOUND;
    }