There is one possible way to get the info (that needs to be verified but will work I guess).
However you’ll need a way to use the Win32 API for this.
Having the Npp handle you can use this Win32 API
::EnumChildWindows(nppData._nppHandle, enumWindowsCB, 0);
and in the enumWindowsCB(HWND hwnd, LPARAM) callback function use
::GetClassName(hwnd, winClassName, _countof(winClassName));
to get the Npp child’s window class type.
When the winClassName is equal to STATUSCLASSNAME you will have the status bar window handle hwnd.
Then you can directly send SB_GETPARTS, SB_GETTEXTLENGTH and SB_GETTEXT messages to the handle to retrieve the info you need.
The status bar part you’ll need to read in your case is STATUSBAR_DOC_TYPE.