how to tell if a string is base64 or not

Question

I have many emails coming in from different sources. they all have attachments, many of them have attachment names in chinese, so these names are converted to base64 by thei

Answer 1

Please note both Content-Transfer-Encoding have base64

Not relevant in this case, the Content-Transfer-Encoding only applies to the body payload, not to the headers.

=?gb2312?B?uLGxvmhlbrixsb5nLnhscw==?=

That's an RFC2047-encoded header atom. The stdlib function to decode it is email.header.decode_header. It still needs a little post-processing to interpret the outcome of that function though:

import email.header
x= '=?gb2312?B?uLGxvmhlbrixsb5nLnhscw==?='
try:
    name= u''.join([
        unicode(b, e or 'ascii') for b, e in email.header.decode_header(x)
    ])
except email.Errors.HeaderParseError:
    pass # leave name as it was

However...

Content-Type: application/vnd.ms-excel;
 name="=?gb2312?B?uLGxvmhlbrixsb5nLnhscw==?="

This is simply wrong. What mailer created it? RFC2047 encoding can only happen in atoms, and a quoted-string is not an atom. RFC2047 §5 explicitly denies this:

• An 'encoded-word' MUST NOT appear within a 'quoted-string'.

The accepted way to encode parameter headers when long string or Unicode characters are present is RFC2231, which is a whole new bag of hurt. But you should be using a standard mail-parsing library which will cope with that for you.

So, you could detect the '=?' in filename parameters if you want, and try to decode it via RFC2047. However, the strictly-speaking-correct thing to do is to take the mailer at its word and really call the file =?gb2312?B?uLGxvmhlbrixsb5nLnhscw==?=!