Cannot upload MODS XML file as media on a node in default setup/sandbox

[kspurgin]

A thing I have heard multiple times in the community is: "Well you aren't throwing any metadata away because you can store the old MODS XML file as a media on your node"

Except, as Islandora comes out of the box/in sandbox, you cannot, because .xml is not an allowed file type on any appropriate media types

The default/basic setup should add .xml as an allowed file type on the File media type.

Related issue: #1137 - more about the mechanics of how one might make this change (in the islandora module or install profile?). This issue is about the fact that this change should be made in the default setup.