uses
 ShellAPI;
function MrsGetFileType(const strFilename: string): string;
var
 FileInfo: TSHFileInfo;
begin
 FillChar(FileInfo, SizeOf(FileInfo), #0);
 SHGetFileInfo(PChar(strFilename), 0, FileInfo, SizeOf(FileInfo), SHGFI_TYPENAME);
 Result := FileInfo.szTypeName;
end;

// Beispiel:
// Example:
procedure TForm1.Button1Click(Sender: TObject);
begin
 ShowMessage('File type is: ' + MrsGetFileType('c:\autoexec.bat'));
end;