Determine if a file is either in JSON or XML format
Well, if you only need to distinguish between JSON and XML formats I'd keep it simple. If the encoding of that string is known to you (or is ASCII or UTF), then looking at the very first char in the file should be enough. Your code looks pythonic to me, but I'd rather do it this way:

def parse_report_file(report_input_file):
    with open(report_input_file) as unknown_file:
        c = unknown_file.read(1)
        if c != '&lt;':
            return 'Is JSON'
        return 'Is XML'


While it is legal for JSON data structures to begin with null, true, false you can avoid those situations if you already know a bit about your data structures.

If you also need to distinguish between JSON, XML and other random file, you should simply strip any whitespace before you look at the "first" char.