diff --git a/common/wildcards_and_files_ext.cpp b/common/wildcards_and_files_ext.cpp
index 084fcafe2b..a147dda47f 100644
--- a/common/wildcards_and_files_ext.cpp
+++ b/common/wildcards_and_files_ext.cpp
@@ -68,40 +68,40 @@ static wxString FormatWildcardExt( const wxString& aWildcard )
 }
 
 
-const wxString SchematicSymbolFileExtension( "sym" );
-const wxString SchematicLibraryFileExtension( "lib" );
-const wxString SchematicBackupFileExtension( "bak" );
+const std::string SchematicSymbolFileExtension( "sym" );
+const std::string SchematicLibraryFileExtension( "lib" );
+const std::string SchematicBackupFileExtension( "bak" );
 
-const wxString VrmlFileExtension( "wrl" );
+const std::string VrmlFileExtension( "wrl" );
 
-const wxString ProjectFileExtension( "pro" );
-const wxString SchematicFileExtension( "sch" );
-const wxString NetlistFileExtension( "net" );
-const wxString ComponentFileExtension( "cmp" );
-const wxString GerberFileExtension( "gbr" );
-const wxString GerberJobFileExtension( "gbrjob" );
-const wxString HtmlFileExtension( wxT( "html" ) );
+const std::string ProjectFileExtension( "pro" );
+const std::string SchematicFileExtension( "sch" );
+const std::string NetlistFileExtension( "net" );
+const std::string ComponentFileExtension( "cmp" );
+const std::string GerberFileExtension( "gbr" );
+const std::string GerberJobFileExtension( "gbrjob" );
+const std::string HtmlFileExtension( "html" );
 
-const wxString LegacyPcbFileExtension( "brd" );
-const wxString KiCadPcbFileExtension( "kicad_pcb" );
-const wxString PageLayoutDescrFileExtension( "kicad_wks" );
+const std::string LegacyPcbFileExtension( "brd" );
+const std::string KiCadPcbFileExtension( "kicad_pcb" );
+const std::string PageLayoutDescrFileExtension( "kicad_wks" );
 
-const wxString PdfFileExtension( "pdf" );
-const wxString MacrosFileExtension( "mcr" );
-const wxString DrillFileExtension( "drl" );
-const wxString SVGFileExtension( "svg" );
-const wxString ReportFileExtension( "rpt" );
-const wxString FootprintPlaceFileExtension( "pos" );
-const wxString KiCadLib3DShapesPathExtension( "3dshapes" );  ///< 3D shapes default libpath
+const std::string PdfFileExtension( "pdf" );
+const std::string MacrosFileExtension( "mcr" );
+const std::string DrillFileExtension( "drl" );
+const std::string SVGFileExtension( "svg" );
+const std::string ReportFileExtension( "rpt" );
+const std::string FootprintPlaceFileExtension( "pos" );
+const std::string KiCadLib3DShapesPathExtension( "3dshapes" );  ///< 3D shapes default libpath
 
-const wxString KiCadFootprintLibPathExtension( "pretty" );   ///< KICAD PLUGIN libpath
-const wxString LegacyFootprintLibPathExtension( "mod" );
-const wxString EagleFootprintLibPathExtension( "lbr" );
+const std::string KiCadFootprintLibPathExtension( "pretty" );   ///< KICAD PLUGIN libpath
+const std::string LegacyFootprintLibPathExtension( "mod" );
+const std::string EagleFootprintLibPathExtension( "lbr" );
 
-const wxString KiCadFootprintFileExtension( "kicad_mod" );
-const wxString GedaPcbFootprintLibFileExtension( "fp" );
-const wxString SpecctraDsnFileExtension( "dsn" );
-const wxString IpcD356FileExtension( "d356" );
+const std::string KiCadFootprintFileExtension( "kicad_mod" );
+const std::string GedaPcbFootprintLibFileExtension( "fp" );
+const std::string SpecctraDsnFileExtension( "dsn" );
+const std::string IpcD356FileExtension( "d356" );
 
 
 const wxString AllFilesWildcard( _( "All files (*)|*" ) );
diff --git a/include/wildcards_and_files_ext.h b/include/wildcards_and_files_ext.h
index bc3fbff5c9..5d666aef0e 100644
--- a/include/wildcards_and_files_ext.h
+++ b/include/wildcards_and_files_ext.h
@@ -46,42 +46,44 @@
  * @{
  */
 
-extern const wxString AllFilesWildcard;
+// Do NOT use wxString for these.  wxStrings are not thread-safe, even when const.  (For the
+// curious the UTF8 cacheing strategy puts iterators in a linked list.  Insertion and removal
+// from the linked list is not thread-safe.)
 
-extern const wxString SchematicSymbolFileExtension;
-extern const wxString SchematicLibraryFileExtension;
-extern const wxString SchematicBackupFileExtension;
+extern const std::string SchematicSymbolFileExtension;
+extern const std::string SchematicLibraryFileExtension;
+extern const std::string SchematicBackupFileExtension;
 
-extern const wxString VrmlFileExtension;
-extern const wxString ProjectFileExtension;
-extern const wxString SchematicFileExtension;
-extern const wxString NetlistFileExtension;
-extern const wxString GerberFileExtension;
-extern const wxString GerberJobFileExtension;
-extern const wxString HtmlFileExtension;
+extern const std::string VrmlFileExtension;
+extern const std::string ProjectFileExtension;
+extern const std::string SchematicFileExtension;
+extern const std::string NetlistFileExtension;
+extern const std::string GerberFileExtension;
+extern const std::string GerberJobFileExtension;
+extern const std::string HtmlFileExtension;
 
-extern const wxString LegacyPcbFileExtension;
-extern const wxString KiCadPcbFileExtension;
+extern const std::string LegacyPcbFileExtension;
+extern const std::string KiCadPcbFileExtension;
 #define PcbFileExtension    KiCadPcbFileExtension       // symlink choice
-extern const wxString PageLayoutDescrFileExtension;
+extern const std::string PageLayoutDescrFileExtension;
 
-extern const wxString LegacyFootprintLibPathExtension;
-extern const wxString PdfFileExtension;
-extern const wxString MacrosFileExtension;
-extern const wxString ComponentFileExtension;
-extern const wxString DrillFileExtension;
-extern const wxString SVGFileExtension;
-extern const wxString ReportFileExtension;
-extern const wxString FootprintPlaceFileExtension;
-extern const wxString KiCadFootprintFileExtension;
-extern const wxString KiCadFootprintLibPathExtension;
-extern const wxString GedaPcbFootprintLibFileExtension;
-extern const wxString EagleFootprintLibPathExtension;
-extern const wxString ComponentFileExtension;
-extern const wxString PageLayoutDescrFileExtension;
-extern const wxString KiCadLib3DShapesPathExtension;
-extern const wxString SpecctraDsnFileExtension;
-extern const wxString IpcD356FileExtension;
+extern const std::string LegacyFootprintLibPathExtension;
+extern const std::string PdfFileExtension;
+extern const std::string MacrosFileExtension;
+extern const std::string ComponentFileExtension;
+extern const std::string DrillFileExtension;
+extern const std::string SVGFileExtension;
+extern const std::string ReportFileExtension;
+extern const std::string FootprintPlaceFileExtension;
+extern const std::string KiCadFootprintFileExtension;
+extern const std::string KiCadFootprintLibPathExtension;
+extern const std::string GedaPcbFootprintLibFileExtension;
+extern const std::string EagleFootprintLibPathExtension;
+extern const std::string ComponentFileExtension;
+extern const std::string PageLayoutDescrFileExtension;
+extern const std::string KiCadLib3DShapesPathExtension;
+extern const std::string SpecctraDsnFileExtension;
+extern const std::string IpcD356FileExtension;
 
 /**
  * @}
@@ -98,6 +100,8 @@ extern const wxString IpcD356FileExtension;
  * @{
  */
 
+extern const wxString AllFilesWildcard;
+
 extern wxString ComponentFileWildcard();
 extern wxString PageLayoutDescrFileWildcard();
 extern wxString SchematicSymbolFileWildcard();