app-container: update SPEC on filesets

Originally we thought that this would be used for making correctly
hashed ACI reassembly possible but we realized that if you are storing
the on-disk ACI then you can also store the ordered list too; for later
verification against a key.
Merge pull request #121 from philips/clarify-appc-fileset-files-list

app-container: update SPEC on filesets