FileAssociation

...
uses
  ...
  FileAssoc; //<-- add fileassociation unit here

type

...
    { private declarations }
    assoc: TFileAssociation;
...

procedure TfrmMain.FormCreate(Sender: TObject);
begin
  assoc := TFileAssociation.Create(Self);//<-- create like a regular component
end;

procedure TfrmMain.Button1Click(Sender: TObject);
begin
  assoc.ApplicationName := 'Lazarus IDE';
  assoc.ApplicationDescription := 'RAD for Free Pascal';

  // you can change Extension and Action part for each extension you have

  assoc.Extension := '.lpr';
  assoc.ExtensionName := 'Lazarus Project';
  assoc.ExtensionIcon := '"C:\lazarus\images\lprfile.ico"';

  // full path required, you can use ParamStr(0) to get the path with the .exe name included. The path must be inside quotes if it has whitespace.
  assoc.Action := '"C:\lazarus\lazarus.exe" "%1"'; 
  assoc.ActionName := 'Open';
  assoc.ActionIcon := '"C:\lazarus\images\mainicon.ico"';

  // notice that using RegisterForAllUsers as True requires Administrator Privileges
  // if you want to run without privileges set it to false, but it will register for current user only
  assoc.RegisterForAllUsers:=False;
  if assoc.Execute then
  begin
    ShowMessage('OK');
    assoc.ClearIconCache; //<<-- rebuild icons
  end;
end;

end.

procedure TForm1.FormCreate(Sender: TObject);
var
  s: String;
begin
// if there are parameters
  if ParamCount > 0 then
  begin
// load the first parameter
    s := ParamStr(1);
 
// if is a .txt file
    if ExtractFileExt(s) = '.txt' then
    begin
// load the .txt file into a memo
      Memo1.Lines.LoadFromFile(s);
    end;
  end;
end;

{$IFDEF WINDOWS}
uses Registry, ShlObj;
 
procedure TMainForm.RegisterFileType(ExtName: string; AppName: string);
var
  reg: TRegistry;
begin
  reg := TRegistry.Create;
  try
    reg.RootKey := HKEY_CLASSES_ROOT;
    reg.OpenKey('.' + ExtName, True);
    reg.WriteString('', ExtName + 'file');
    reg.CloseKey;
    reg.CreateKey(ExtName + 'file');
    reg.OpenKey(ExtName + 'file\DefaultIcon', True);
    reg.WriteString('', AppName + ',0');
    reg.CloseKey;
    reg.OpenKey(ExtName + 'file\shell\open\command', True);
    reg.WriteString('', AppName + ' "%1"');
    reg.CloseKey;
  finally
    reg.Free;
  end;
  SHChangeNotify(SHCNE_ASSOCCHANGED, SHCNF_IDLIST, nil, nil);
end;
{$ENDIF}

var
   reg: TRegistry;
       
begin
  {$IFDEF WINDOWS}
  reg := TRegistry.Create;
  try
    reg.RootKey := HKEY_CLASSES_ROOT;
    if not reg.KeyExists('ext' + 'file\shell\open\command') then
      RegisterFileType('ext', ExtractFilePath(Application.ExeName) + 'my_app.exe');
  finally
    reg.Free;
  end;
 
 if ParamCount > 0 then
  begin
    s := ParamStr(1);
    if ExtractFileExt(s) = '.ext' then
      LoadFile(s);
  end;
  {$ENDIF}