I had this problem just a while ago.
Problem: I couldn't paste any file from a site to this very space I'm typing now. It was quite long so didn't dare to retype them so I searched the net & found this answer:
Solution: Try hitting the EDIT HTML first before pasting it. It worked for me.
This problem usually occurs when you see the the box that contains the title of your blog post tends to overlap the box for the body of your message -- sort of like a warning sign that you won't be able to paste.
Through constant research I've found out that this sort of problem happenst to Internet Explorer users. Before I encountered the sais problem I have updated my IE to IE 8 so I guess there's a connection there. Some said in some forums that they don't encounter such problem in Firefox!
Source: I found this helful advice from this forum from dwillright! Thank you so much whoever you are!