Extracts/decodes a message section from the supplied filename.
The contents of the section will be decoded according to their transfer encoding - base64, quoted-printable and uuencoded text are supported.
mimemail
A valid MIME resource, created with mailparse_msg_create().
filename
Can be a file name or a valid stream resource.
callbackfunc
If set, this must be either a valid callback that will be passed the extracted section, or NULL to make this function return the extracted section.
If not specified, the contents will be sent to "stdout".