Ive had this error reported to me a few times by different users but never been able to sort it. It happens when adding attachments, this user has reported it happens with firefox but not with chrome using the same file. he has also reported that if he disables flash in firefox he can successfully upload the file. I have tried Flash Uploader disabled/enabled it doesn't make a difference still get error. he is using firefox 43.0.4 the same as me but im having no problems.