I am having the same issue as described here (http://wordpress.org/support/topic/not-resolved-404-error-on-uploaded-images?replies=4#post-5000336) and unfortunately the issue is *not* resolved.
Since upgrading to WP 3.8, I am unable to upload any images. The upload process itself looks like it is working, but then the image does not appear. This is affecting all of the WP sites that I updated to 3.8.
FYI, I have already tried deactivating ALL plugins and switching to the default theme. I also cleared caches and reset my permalinks.
These steps did not fix the problem.
My .htaccess files do *not* contain: <Files *.php>deny from all</Files>
Assistance with this would be much appreciated!!