Thanks for your inquiry. While using the latest version of Aspose.Words i.e. 11.10.0, I managed to reproduce this exception on my side. I have logged this issue in our bug tracking system. The issue ID is WORDSNET-7493. Your request has also been linked to this issue and you will be notified as soon as it is resolved. Sorry for the inconvenience.
PS: I was unable to reproduce this exception during loading your 'Unknown file format1.txt' file into Aspose.Words' DOM. I would suggest you please upgrade to the latest version of Aspose.Words i.e. 11.10.0. You can download it from the following link:
Thanks for yuor inquiry. Unfortunately, this issue is not resolved yet. Our development team has completed the analysis of this issue and the root cause has been identified. We will be sure to inform you as soon as this issue is resolved. We apologize for any inconvenience.