WXR xml files are being imported as a TXT files and showing up under ‘Media’

DomDocument is a PHP extension. It is either not installed or is disabled. You will need to install that module and/or enable it.

You will need server level access to correct it. That is, you will need to be able to configure PHP. How you do that depends on what kind of server you have, so I am not sure what else to say at this point. Perhaps if you were to edit your question to include server details…