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.
A valid MIME resource, created with mailparse_msg_create().
Can be a file name or a valid stream resource.
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".
callbackfunc is not NULL returns TRUE on
callbackfunc is set to NULL, returns the
extracted section as a string.
Returns FALSE on error.