Improve input validation

[rexxars]

If the received file field does not have the expected value, instead of giving a meaningful error, it responds with:

{
  "percent": 100,
  "status": "failed",
  "message": "can't convert Symbol into Integer"
}

Info:

#<TypeError: can't convert Symbol into Integer>
  /srv/tiramisu/current/api/v1/images.rb:56:in `[]'
  /srv/tiramisu/current/api/v1/images.rb:56:in `block (2 levels) in <class:TiramisuV1>'