Added a test case for the display_for_field function when a FileField is passed.

[Antoliny]

def display_for_field(value, field, empty_value_display):
    from django.contrib.admin.templatetags.admin_list import _boolean_icon
    ...
    elif isinstance(field, (models.IntegerField, models.FloatField)):
        return formats.number_format(value)
    elif isinstance(field, models.FileField) and value:
        return format_html('<a href="{}">{}</a>', value.url, value)
    ...

There is no test case for the display_for_field function when a FileField object is passed. --> ​admin_utils test code

When the FileField handling was added in the above function, I found that the test was conducted indirectly through the ModelAdmin readonly_fields attribute. --> ​test code

However, I believe it would be better to have a direct test case for when a FileField is passed, considering that display_for_field function is globally accessible.