def _guess_extension(content_type):
    """
    Python's `mimetypes.guess_extension` is no use because it simply returns
    the first of an unordered set. We use the same set of media types here,
    but take a reasonable preference on what extension to map to.
    """
    return {
        "application/javascript": ".js",
        "application/msword": ".doc",
        "application/octet-stream": ".bin",
        "application/oda": ".oda",
        "application/pdf": ".pdf",
        "application/pkcs7-mime": ".p7c",
        "application/postscript": ".ps",
        "application/vnd.apple.mpegurl": ".m3u",
        "application/vnd.ms-excel": ".xls",
        "application/vnd.ms-powerpoint": ".ppt",
        "application/x-bcpio": ".bcpio",
        "application/x-cpio": ".cpio",
        "application/x-csh": ".csh",
        "application/x-dvi": ".dvi",
        "application/x-gtar": ".gtar",
        "application/x-hdf": ".hdf",
        "application/x-latex": ".latex",
        "application/x-mif": ".mif",
        "application/x-netcdf": ".nc",
        "application/x-pkcs12": ".p12",
        "application/x-pn-realaudio": ".ram",
        "application/x-python-code": ".pyc",
        "application/x-sh": ".sh",
        "application/x-shar": ".shar",
        "application/x-shockwave-flash": ".swf",
        "application/x-sv4cpio": ".sv4cpio",
        "application/x-sv4crc": ".sv4crc",
        "application/x-tar": ".tar",
        "application/x-tcl": ".tcl",
        "application/x-tex": ".tex",
        "application/x-texinfo": ".texinfo",
        "application/x-troff": ".tr",
        "application/x-troff-man": ".man",
        "application/x-troff-me": ".me",
        "application/x-troff-ms": ".ms",
        "application/x-ustar": ".ustar",
        "application/x-wais-source": ".src",
        "application/xml": ".xml",
        "application/zip": ".zip",
        "audio/basic": ".au",
        "audio/mpeg": ".mp3",
        "audio/x-aiff": ".aif",
        "audio/x-pn-realaudio": ".ra",
        "audio/x-wav": ".wav",
        "image/gif": ".gif",
        "image/ief": ".ief",
        "image/jpeg": ".jpe",
        "image/png": ".png",
        "image/svg+xml": ".svg",
        "image/tiff": ".tiff",
        "image/vnd.microsoft.icon": ".ico",
        "image/x-cmu-raster": ".ras",
        "image/x-ms-bmp": ".bmp",
        "image/x-portable-anymap": ".pnm",
        "image/x-portable-bitmap": ".pbm",
        "image/x-portable-graymap": ".pgm",
        "image/x-portable-pixmap": ".ppm",
        "image/x-rgb": ".rgb",
        "image/x-xbitmap": ".xbm",
        "image/x-xpixmap": ".xpm",
        "image/x-xwindowdump": ".xwd",
        "message/rfc822": ".eml",
        "text/css": ".css",
        "text/csv": ".csv",
        "text/html": ".html",
        "text/plain": ".txt",
        "text/richtext": ".rtx",
        "text/tab-separated-values": ".tsv",
        "text/x-python": ".py",
        "text/x-setext": ".etx",
        "text/x-sgml": ".sgml",
        "text/x-vcard": ".vcf",
        "text/xml": ".xml",
        "video/mp4": ".mp4",
        "video/mpeg": ".mpeg",
        "video/quicktime": ".mov",
        "video/webm": ".webm",
        "video/x-msvideo": ".avi",
        "video/x-sgi-movie": ".movie",
    }.get(content_type, "")