Some MIME types can be intuitive - application/pdf for PDF files. However, not all MIME types are that easy to get exactly right - application/vnd.ms-excel for xls or Excel files.
The following are different ways, using the command line, to identify the MIME type of a file.
$ xdg-mime query filetype somefile.xls application/vnd.ms-excel
$ file -b --mime-type somefile.xls application/vnd.ms-excel
$ gvfs-info -a standard::content-type somefile.xls attributes: standard::content-type: application/vnd.ms-excel