Hi,
I've isolated the cause to some kind of interference from the sarrissa xmlhttp script that we use for the friendly url suggest on the blog edit page, it doesn't cause any script errors but seems to break the jquery postback that posts the file.
sarrisa is kind of an old javascript that I've been meaning to replace with similar functionality from jquery. It isn't a huge job but it isn't a trivial fix either.
So this problem is something I will have to fix for the next release of mojoPortal. In the meantime all I can suggest is use a workaround such as a different browser mode in IE or a different browser like Chrome of Firefox when uploading media attachments in the blog.
Thanks again for bringing this to my attention. I will post again here once it is fixed in our source code repository.
Best,
Joe