How to upload different files for each user thread in JMeter

The way to have each individual JMeter thread (user) upload a different file to a remote web server is to add entries to the "User Parameters" module.

1) In your thread-group, Add Pre Processors > User Parameters to Thread Group
2) To add variables specific to each user, see: http://jakarta.apache.org/jmeter/usermanual/component_reference.html#User_Parameters