You have been trying to send files continuously in the past few hours but your all efforts are in vain. Could you know what is the exactly reason behind this problem, why this is happening? If no, then immediately follow my steps below where you know how to solve this particular problem.
1) Guys have you checked well your browsers in which you are using your icloud mail? Why I am asking this question? Because icloud mail is best working in some of the supported web browsers such as: internet explorer, Google chrome, safari, Mozilla Firefox and so on. Before sending and receiving the attachment kindly prefer these web browsers and hope so your problem will solve now. If no, then find the next solution below.
2) If you exceed the file size then surely you are not able to send the attachment. So whenever you have to send any attachments please double check the file size of that attachment.
3) One more issue is, internet connection problem so that users didn’t send the file and attachment. Kindly contact to internet service provider or network administrator and ask them for instant solution regarding this issue.