**Description**

Files are not part of the templated fields in the `EmailOperator` https://airflow.apache.org/docs/stable/_modules/airflow/operators/email_operator.html

Whle in fact file names should also be templated .

**Use case / motivation**
We want to store files accordingly to the DAGs execution date and further send them to email recipients like so:

```
send_report_email = EmailOperator(
        ....
        files=[
            "/tmp/Report-A-{{ execution_date.strftime('%Y-%m-%d') }}.csv",
            "/tmp/Reportt-B-{{ execution_date.strftime('%Y-%m-%d') }}.csv"
        ]
```

airflow/operators/email.py
"""
    Sends an email.
    :param to: list of emails to send the email to. (templated)
    :type to: list or string (comma or semicolon delimited)
    :param subject: subject line for the email. (templated)
    :type subject: str
    :param html_content: content of the email, html markup
        is allowed. (templated)
    :type html_content: str
    :param files: file names to attach in email (templated)
    :type files: list
    :param cc: list of recipients to be added in CC field
    :type cc: list or string (comma or semicolon delimited)
    :param bcc: list of recipients to be added in BCC field
    :type bcc: list or string (comma or semicolon delimited)
    :param mime_subtype: MIME sub content type
    :type mime_subtype: str
    :param mime_charset: character set parameter added to the Content-Type
        header.
    :type mime_charset: str
    """
    template_fields = ('to', 'subject', 'html_content', 'files')
    template_fields_renderers = {"html_content": "html"}
    template_ext = ('.html',)
    ui_color = '#e6faf9'
    @apply_defaults
    def __init__(