Convert Xmind8 to Xmind21 for use on browser

I have a bunch of Xmind8 files that need to be shown on web (browser) and shared. Right now, when I try to open the files, it shows me an error "This file failed to open.". The file opens perfectly fine on Xmind8 app on my macbook, but not on Xmind21. The intention is to upload all these files on Xmind.works so that it is sharable.

Is there some software/sdk that converts Xmind8 to Xmind21?

These files are stylized and use markers, and so exporting to word and then importing is not an option