how do I get the application that opens/executes files with extension ".???"

I wrote a function for this purpose:
it returns the application with the full path to where its located, empty if not found

Function GetAssociation(Const Fileext: String): String;
Var
  FileClass, Tmp: String;
  Reg: TRegistry;
Begin
  Result := '';
  Reg := TRegistry.Create(KEY_EXECUTE); //read only please
  Reg.RootKey := HKEY_CLASSES_ROOT;
  FileClass := '';
  If Reg.OpenKeyReadOnly(Fileext) Then
  Begin
    FileClass := Reg.ReadString('');  //read the default
    Reg.CloseKey;
  End;
  If FileClass <> '' Then Begin
    If Reg.OpenKeyReadOnly(FileClass + '\Shell\Open\Command') Then
    Begin
      Result := Reg.ReadString('');
      Reg.CloseKey;
    End;
  End;
  Reg.Free;
  If (Length(Result)>0) Then
    If (Result[1]='"') Then
    Begin
      Delete(Result, 1, 1);
      Result:=Copy(Result, 1, Pos('"', Result)-1)
    End
    Else
    Begin
      Tmp:=ExtractFileExt(Result);
      If (Pos(' ', Tmp)>0) Then
        Tmp:=Copy(Tmp, 1, Pos(' ', Tmp)-1);
      Result:=Copy(Result, 1, Pos(Tmp, Result)+Length(Tmp))
    End
End;

...ok... then as an example: drop a TEdit and a TButton on your form, put the extension that you need on the TEdit, like:
TEdit.Text:='.ZIP'
(don't forget to include the "." before the extension)

then on the onclick event of the button:

Procedure TForm1.Button2Click(Sender: TObject);
Begin
  ShowMessage(GetAssociation(Edit1.Text))
End;

that's it

keep up coding

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EberSys