Fix mimi to handle executable with arguments

Hello,
I want to be able to pass executable with arguments to xdg-open, right now mimi can't guess right the file type.

For example I want to be able to do this:
`/usr/bin/xdg-open /path/to/executable --arguments 'foo'`

This commit fixes mimi to check if the first argument is executable and run `file` only for it.