Support sending attachments without filename

[leplatrem]

Use case: from kinto-http.js, being able to attach a file coming from a dataURI (without filename).

def save_file(content, request, randomize=True, gzipped=False):
    folder_pattern = request.registry.settings.get('attachment.folder', '')
    folder = folder_pattern.format(**request.matchdict) or None

    # Read file to compute hash.
    if not isinstance(content, cgi.FieldStorage):
        error_msg = 'Filename is required.'
        raise_invalid(request, location='body', description=error_msg)