Warn the user of "harmless" filetypes
mat2 currently considers plain application/xml files as being free of metadata, when it's really entirely possible that an XML file could have metadata in a schema that we don't know about. We can't support every possible schema, of course, but currently the tool runs the same way whether it's actually removing metadata or it isn't, and this could result in someone getting a false sense of security.
I'm imagining someone wanting to leak some in-house, totally undocumented schema in a hurry; knowing what metadata is; but not knowing a lot about how file formats really work. That person might run mat2 on their file and think it's clean. We should tell them not to assume this.