helper function to get base64Binary attachment data from emails

[Alexander Kreutz]

If you fetch an email message using imap:fetch-message and this email
contains a binary attachment, the attachment data in the email:content element
in not of type xs:base64Binary. It is a string with the base64 encoded data including line breaks and spaces.

To use the binary data it must first be manually cleaned with an expression like:

fn:replace($attachment/text(),"[^a-zA-Z0-9\+/=]","") cast as xs:base64Binary

There should be a helper function to receive the base64Binary from an email attachment.