Unnecessary file fields extraction

I have a CT cv_book_display, which has three fields of file type.
They are: field_files, field_book_cover, field_service_files.
At the index configuration page i check only attachments_field_files field. However, with enabled Debug extraction of attachments i see, that files, related to field_book_cover are being extracted.
Am i missing something or is this really a bug?

I've taken a peek at this function which is called right before getFileContent($file) function inside alterItems(array &$items).
I suppose, this is the place, where we should check if field was selected. But i'm not sure. Could anyone provide me with a clue?

Update

I suppose, we have to filter fields, which will be used in alterItems(array &$items) function. Check if field is inside search index and only then go to the getFilecontent function.