Can I send really large files over ActiveMQ
The answer is yes.
If you are using ActiveMQ 4.2 or later we highly recommend you use Blob Messages which implements an out of band transport of the messages; it allows the files to be hosted on external http/ftp sites if required and can support either direct publisher <-> subscriber communication or publisher -> broker/file server -> consumer messaging.
For 4.1 or ealier large file transfer is achieved using JMS Streams.
Normally the JMS API expects the entire JMS messsage to reside in the client side memory; however using Blob Messages or JMS Streams allows you to send and receive arbitrarily large files with very low RAM overhead.