uses Registry;

function RegisterFileTypeCommand(fileExtension, menuItemText, target: string) : boolean;
var
reg: TRegistry;
fileType: string;
begin
result := false;
reg := TRegistry.Create;
with reg do
try
RootKey := HKEY_CLASSES_ROOT;
if OpenKey('.' + fileExtension, True) then
begin
fileType := ReadString('') ;
if fileType = '' then
begin
fileType := fileExtension + 'file';
WriteString('', fileType) ;
end;
CloseKey;
if OpenKey(fileType + '\shell\' + menuItemText + '\command', True) then
begin
WriteString('', target + ' "%1"') ;
CloseKey;
result := true;
end;
end;
finally
Free;
end;
end;

function UnRegisterFileTypeCommand(fileExtension, menuItemText: string) : boolean;
var
reg: TRegistry;
fileType: string;
begin
result := false;
reg := TRegistry.Create;
with reg do
try
RootKey := HKEY_CLASSES_ROOT;
if OpenKey('.' + fileExtension, True) then
begin
fileType := ReadString('') ;
CloseKey;
end;
if OpenKey(fileType + '\shell', True) then
begin
DeleteKey(menuItemText) ;
CloseKey;
result := true;
end;
finally
Free;
end;
end;
**************************

//Register ... use
RegisterFileTypeCommand('pas','Preview in Notepad','C:\WINDOWS\Notepad.exe') ;

//Unregister
UnRegisterFileTypeCommand('pas','Preview in Notepad') ;