Sorry, I can not reproduce this.
Is that a new account, which never used "Save" before?
Do you have to wait between uploading attachments for some time?
Which Browser do you use?
Does this occur with big attachments only?

Hello,
I use Mozilla Firefox 39 and Google Chrome 43.
This problem seems not happen with all attachment type.
I sent a mail with four attachments jpg pictures. This mail is arrived in the receiver's box with all attachments but in the sender's box I've two mail in Drafts's directory with two attachments. I waited ten minutes to see if mails will be remove but they are still here.

Attachment's size are between 70Ko and 200Ko.
Please find in attachment the screenshot of

senders box.png is a picture of the mail with attachments in sender's box.
senders draft directory.png is a picture of mails with attachments in draft's directory of the sender's box.
receivers box.png is a picture of the mail with all attachments int receiver mail box.

Best regards,
Tifenn LE GOFF

Suspending for now, unable to reproduce with v2 and v3 with 100 attachments added at once. If you can reproduce the problem please reopen this ticket and provide more evidences.