Can I prevent user from importing a non-existant or incorrect file in an import operator?

Question

Losing my Blender flower on this one :)

I'm using an operator to select the filepath of a .txt file.

import bpy
class SelectTxtFile(bpy.types.Operator):
    bl_idname = "file.select_txt"
    bl_label = "Select TXT File"
    bl_options = {"REGISTER", "UNDO"}
    filepath: bpy.props.StringProperty(subtype="FILE_PATH")
    filter_glob: bpy.props.StringProperty(default="*.txt", options={"HIDDEN"})
def execute(self, context):
    print(self.filepath)
    return {"FINISHED"}

def invoke(self, context, event):
    context.window_manager.fileselect_add(self)
    return {"RUNNING_MODAL"}


def register():
    bpy.utils.register_class(SelectTxtFile)
if name == "main":
    register()
bpy.ops.file.select_txt('INVOKE_DEFAULT')

I don't want the user to be able to validate the operator (Blue button) if the file they selected isn't a valid .txt file.

I want the button to be clickable only when :

• The user selected a file and it exists : os.path.isfile(path) returns True
• The extension is .txt : os.path.splitext(path) returns ".txt"

I would like to insert this line somewhere :

layout.enabled = os.path.isfile(self.filepath) and os.path.splitext(self.filepath)[1] == ".txt"

I'd like it to be grayed out if the conditions are not met. But I can't access this particular layout button because it's apparently managed by the window manager.

How can I achieve that ?