Actually no, but i got it working using a different approach.
Uploading from servlet did not work. Maybe one day i'll take a look with fiddler. (CA Support told me not to download from it)
RESTful API. My current 12.9 installation needs a patch to get the downloadAttachment method working. My installation is currently ultra stable so i do not want to compromise this with a patch.
I've done the old, creepy way. Sharing the attachment folder with a specific user. Copy it with PAM, decompress it, rename it and send it.