I already reported this problem on the tracker with all the necessary information:
http://trac.roundcube.net/ticket/1490411
But I think I better discuss this here as it's more likely a call for help/support. Probably an nginx or php config issue. Not sure.
FYI, `phpinfo()` confirms that I have this:
Server API FPM/FastCGI
file_uploads On
max_file_uploads 20
max_execution_time 120
upload_max_filesize 100M
post_max_size 120M
So, these PHP ini values look right to me. Must be something else. Any clues?
Make sure the Roundcube temp directory is writable.