<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def find_output_with_tag(self, tag):
 """ Find all files who have tag in self.tags """

        # Enforce upper case
        tag = tag.upper()
        return FileList([i for i in self if tag in i.tags])