Fix type-dependent return from getBytes()

 * From the LA audit,

   "Concern: There is a type-dependent return from Cryptocat.getBytes()
   which actually causes calls to Cryptocat.encodedBytes(1, ...) to throw
   an exception."
Rename fileSize to maximumFileSize

 * From the LA audit,

   "Cryptocat.fileSize should be named Cryptocat.maximumFileSize."
Merge pull request #625 from arlolra/la

Some concerns from the LA audit