AnyEvent: extend send_file with content-type

AnyEvent: extend send_file with content-type

for pmg, we need to send temporary files (for the attachment quarantine),
but we cannot know beforehand what content-type it is, so we
optionally give it to send_file_start

we give a hash with the open filehandle and the content-type in that case

this also removes the unnecessary open on the filename, since we open
it in send_file_start anyway...

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>