Change collection set file structure

[maxnth]

Currently the files property of a collection set object just contains an array of all files in the collection set without any additional information regarding the file type or "extension type". In the future the request should return a more structured format like:

...
{ files: [
  { "content-type": "image", "type": "nrm", "file:": "some_page.nrm.png"},
  { "content-type": "image", "type": "bin", "file:": "some_page.bin.png"},
  { "content-type": "xml", "type": null, "file:": "some_page.xml"},
]}
...

or something similar to allow starting training only on e. g. grayscale images.

[maxnth]

Was added to dev branch