Is there a way to convert the uploaded excel file to CSV using django pre_save signal?
Yes, you could do it in pre_save, but preferably just do this in the Django form or DRF serializer that handles the upload. That way you can bubble up conversion errors to the user as validation errors, etc.
Assuming the field in your model is upload, you'd use

clean_upload(self, file) in Django forms.
validate_upload(self, file) in DRF serializers.

Just return a new ContentFile() with your converted content from one of those.
I would like to know if it is possible to convert the user selected excel file to CSV using django pre_save signals. Suppose a user uploads 'test.xlsx', I would like to convert that file automatically using the pre_save signal. Once the user uploads the file, it should actually be 'test.csv' in my FileField.