I used to find the extensions mapping from System`ConvertersDump`$extensionMappings
, which gives a mapping like "{"*.3ds" -> "3DS"...}". This seems in V13 it is removed, though some related contexts remain defined with no results:
Select[Names["System`*`*"],
StringMatchQ[#, "*extension*", IgnoreCase -> True] &]
(* {"System`Dump`ensureLibraryFileExtension",
"System`Dump`extension",
"System`ConvertersDump`extensions",
"System`ConvertersDump`extensions$",
"System`Dump`extension$",
"System`ConvertersDump`Utilities`Private`fileExtension",
"System`ConvertersDump`fileExtensions",
"System`FEDump`forceFileExtension",
"System`ConvertersDump`GetExtensionMappings",
"System`Dump`LibraryExtension"} *)
I checked all the above contexts, and none gives what I want.