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There are two likely causes of 500 Errors on file uploads - both of them related to servers settings. One is the upload_max_filesize in the php.ini file; the other an Apache MaxRequestLen setting.
There is a Help page on HowToForge.com that shows how to fix both of these problems. You may need to check with your web host about how to edit the settings on your site - neither of these are Joomla! or ChronoForms settings.
The default value for the Apache MaxRequestLen setting has been reduced from 1Gb to 128kb (131072 bytes) in Apache v2.3.6 development branch and the v2.4 stable branch released in 2013 so this may cause unexpected problems after server upgrades.
