This is very general problem which is face by mostly users. Unfortunately this problem has occurred due to technical glitches. But if you want reliable solution regarding this problem, finds the solution here.
1) Have you check your attachment’s file size before sending or receiving it? If no, then quickly check it, because if you exceed its limit size the definitely you are not allow to upload the attachment.
2) If your attachment considered any type of special symbols or characters such as: hash tag, dollar sign etc then outlook prevents you to upload the attachment. So, keep check your special characters before uploading any attachment.
3) Which browser you are using your outlook? Why I am asking this question, because outlook is perfectly working in some of the best supported browsers like: safari, internet explorer, Google chrome etc.
4) Sometime internet problem creates trouble for you that’s why users are not uploading the attachment. If is it so, then quickly resolve this problem by contacting your internet service provider.
5) If there is any kind of antivirus issue, so immediately turn off your antivirus and again try to send your files. May be it works again.