Fix mime checking when unix util "file" version <5.

This is potentially relevant in #1506, but it doesn't really affect the current choices, rather, it merely ensures that the current code works even on older versions of the `file` command on unix/linux.

For versions prior to 5, the `-i` option provides the mime type. That option doesn't exist in version 5 and has been replaced with the `--mime` that Paperclip was using by default.