TIBCO ActiveMatrix Adapter for Files (Unix/Win) does not have any builtin method to get memory usage. You can use an OS level command to get memory usage for the 'adfilesagent' process. A custom script or custom hawkrule base needs to be created using the OS level command to collect the data.

Another simpler option is, if you are deploying TIBCO ActiveMatrix Adapter for Files (Unix/Win) under TIBCO ActiveMatrix Administrator, then you can 

1). First click on Resource Management->Machines->Select File adapter running machine. This will give you Machine level CPU/Memory usage.

2). Click on the Process tab. You will see all processes running on that machine and you can see how much CPU/memory is used by the file adapter process.

Note that file adapter 7.0.0 is first Java based release which means you can specify max heap size for the adapter process.