The following example demonstrates retrieving the file associated with a file extension. Example:

uses
{$IFDEF WIN32}
  Registry; {We will get it from the registry}
{$ELSE}
  IniFiles; {We will get it from the win.ini file}
{$ENDIF}
{$IFNDEF WIN32}
  const MAX_PATH =  44;
{$ENDIF}
function GetProgramAssociation (Ext : string) : string;
var
{$IFDEF WIN32}
  reg: TRegistry;
  s : string;
{$ELSE}
  WinIni : TIniFile;
  WinIniFileName : array[0..MAX_PATH] of char;
  s : string;
{$ENDIF}
begin
{$IFDEF WIN32}
  s := '';
  reg := TRegistry.Create;
  reg.RootKey := HKEY_CLASSES_ROOT;
  if reg.OpenKey('.' + ext + '\shell\open\command',
                 false) <> false then begin
  {The open command has been found}
    s := reg.ReadString('');
    reg.CloseKey;
  end else begin
  {perhaps thier is a system file pointer}
    if reg.OpenKey('.' + ext,
                   false) <> false then begin
      s := reg.ReadString('');
      reg.CloseKey;
      if s <> '' then begin
     {A system file pointer was found}
        if reg.OpenKey(s + '\shell\open\command',
                       false) <> false then
     {The open command has been found}
          s := reg.ReadString('');
        reg.CloseKey;
      end;
    end;
  end;
 {Delete any command line, quotes and spaces}
  if Pos('%', s) > 0 then
    Delete(s, Pos('%', s), length(s));
  if ((length(s) > 0) and
      (s[ ] = '"')) then
    Delete(s,  ,  );
  if ((length(s) > 0) and
      (s[length(s)] = '"')) then
    Delete(s, Length(s),  );
  while ((length(s) > 0) and
         ((s[length(s)] = #32) or
          (s[length(s)] = '"'))) do
    Delete(s, Length(s),  );
{$ELSE}
  GetWindowsDirectory(WinIniFileName, sizeof(WinIniFileName));
  StrCat(WinIniFileName, '\win.ini');
  WinIni := TIniFile.Create(WinIniFileName);
  s := WinIni.ReadString('Extensions',
                          ext,
                          '');
  WinIni.Free;
 {Delete any command line}
  if Pos(' ^', s) > 0 then
    Delete(s, Pos(' ^', s), length(s));
 {$ENDIF}
  result := s;
end;
procedure TForm .Button Click(Sender: TObject);
begin
  ShowMessage(GetProgramAssociation('gif'));
end;