I am having issues displaying XML files in Ark, since Ark tries to render the XML and lossing most of the information within the XML. Is there a way to change the mimetype of Ark so that it shows it on plain text?

Open System Settings > File Associations.
As a filter provide "xml", then find application/xml in the search results.

On the Embedded tab, ensure "Embedded Advanced Text Editor (katepart)" is at the top, and apply.
You may need to restart Ark for this to apply.