As soon as you say wxFormBuilder, my eyes glaze over, as I’ve always coded directly, so this may or may not be of help.

You need to Bind to an EVT_DROP_EVENT to know that something has been dropped and have set a target to process what got dropped.

Here is an example, sadly no wxFrameBuilder involved:

import wx
import wx.lib.newevent

drop_event, EVT_DROP_EVENT = wx.lib.newevent.NewEvent()

class FileDropTarget(wx.FileDropTarget):
    def __init__(self, obj, multiple_allowed=False):
        wx.FileDropTarget.__init__(self)
        self.obj = obj
        self.multiple_allowed = multiple_allowed

    def OnDropFiles(self, x, y, filenames):
        filename = filenames[0]
        evt = drop_event(data=filename)
        wx.PostEvent(self.obj, evt)
        return True


class MyFrame(wx.Frame):
    def __init__(self, parent, title):
        super(MyFrame, self).__init__(parent, style = wx.DEFAULT_FRAME_STYLE,
                            title=title, size=(500, 500))
        self.panel = wx.Panel(self, size = (-1, -1))
        self.bmp = wx.Bitmap()
        self.bitmap = wx.StaticBitmap(self.panel, -1, self.bmp)

        self.file_drop_target = FileDropTarget(self, multiple_allowed=False)
        self.SetDropTarget(self.file_drop_target)
        self.Bind(EVT_DROP_EVENT, self.OnDropEvent)

    def OnDropEvent(self, event):
        drop_filename = event.data
        self.bitmap.SetBitmap(wx.Bitmap(drop_filename, wx.BITMAP_TYPE_ANY))
        self.Refresh()
        return

if __name__ == "__main__":
    app = wx.App()
    MainFrame = MyFrame(None, title = "Drop Image")
    MainFrame.Show()
    MainFrame.Centre()
    app.MainLoop()

After checking the drag_accept_files property for the panel you need to define the event handler OnDropFiles.

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Now you can generate the code with wxFormBuilder.

In the python source for your form you have to program the logic for the defined event handler. In my example on_panel_dnd_drop_files. It just prints the files and directories received.

def on_panel_dnd_drop_files( self, event ):
    for path in event.Files:
        if os.path.isfile(path):
            print(f'dnd file: {path}')
        elif os.path.isdir(path):
            print(f'dnd directory:  {path}')

You can drag and drop files as well as directories as well as a combination. Here is the result of drag and dropping some files and directories from my windows directory:

    dnd file: C:Windowsbfsvc.exe
    dnd directory:  C:WindowsWaaS
    dnd directory:  C:WindowsWeb
    dnd directory:  C:WindowsWinSxS
    dnd file: C:Windowsampa.exe
    dnd file: C:WindowsATTODiskBenchmark.INI

As you can see in the example event handler, you can use os.path.isfile and os.path.isdir to distinguish between files and directories.