XMI is a subset of XML, so a text-editor should work, and depending on the type of XMI files, XML viewers might work. Firefox works on some files, for instance. okular has plugins for a lot of files -- you might try that if you haven't.
If you interested are interested in doing more than viewing, you might want to look into converters, so that another tool can do the job. (So a standard XML tool would work.)
On a slightly more picky note, I see that https://stackoverflow.com/questions/6199504/convert-an-xmi-file-to-xml-file-using-python indicates that it is preferable to call an XMI to XML conversion an XML "transformation", since XMI is (a type of) XML.
conglomerate are two XML editors in the repositories (and software center), which you might try if you haven't.