I found lots of examples in a Google search explaining that any special characters before the <?xml… will result in this error. Check your filename that it doesn't have any special characters in front, that you are selecting the file and not the folder it is in during the import, that you properly setup the model reflection in your template for the mapping and you don't have special characters in the file, and that your excel file is properly encoded to the correct character set like UTF-8. Beyond that I would need to see your template configuration and the source file.
The importer is called "Excel Importer" but does the file have to be in an XML format? I've tried saving the file as Excel XML but that didn't work either. The filename has no special characters in it.
If it has to be formatted as XML then it's even worse than I thought. One of the Mendix competitors allows for direct import via "Import from Excel". Why can't Mendix have that? Seems like we're in the 21st century now.