<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def delete_attachment(request, link_field=None, uri=None):
 """Delete existing file and link."""

    if link_field is None:
        link_field = "record_uri"
    if uri is None:
        uri = record_uri(request)

    # Remove file.
    filters = [Filter(link_field, uri, core_utils.COMPARISON.EQ)]
    storage = request.registry.storage
    file_links, _ = storage.get_all("", FILE_LINKS, filters=filters)
    for link in file_links:
        request.attachment.delete(link['location'])

    # Remove link.
    storage.delete_all("", FILE_LINKS, filters=filters, with_deleted=False)