Files with MIME entities generated and encryted with testkeys/sender.crt
in real MUA(look into files for User-Agent)

* simple-txt-enc.eml
    simple plain/text message with 'test' content

* with-text-attachment.eml
    multipart message with a text part and a text attachment.
    Content of the text is 'test'. Name of the attachment
    is 'attachment.txt' and content is 'text attachment'.

* with-bin-attachment.eml
    multipart message with a text part and a binary attachment.
    Content of the text is 'test'. Name of the attachment
    is 'attachment.bin' and content is 32 random bytes.