When you ask Kloudless to retrieve the files from an account, using: GET /v0/accounts/{account_id}/folders/{id}/contents/, it only lists the actual files, there are no thumbnail files.
So you cannot use the get files contents:GET /v0/accounts/{account_id}/files/{id}/contents/ because it needs a specific file id for the thumbnail file, but you don't get that because none are listed in the preview call.
So how do you retrieve thumbnails for the files?
2016-09 Update: A thumbnails endpoint (docs) is now available for select services. The prior SO answer has been preserved below as it describes the File Download endpoint which is valuable to obtain the file contents for services that do not yet support obtaining thumbnails for.
At the current time the Kloudless API does not support returning thumbnails for files stored in users' cloud storage accounts.
The request that you are making:
GET /v0/accounts/{account_id}/files/{id}/contents/
is a download request which fetches the full contents of the file.
The file ID can be obtained from the objects listed in the children request which you referenced before:
GET /v0/accounts/{accounts_id}/folders/{id}/contents/
This will return a list of file/folder objects which have the ID of the resource as well as other metadata. The ID in the returned file objects can be used in the download request to fetch the contents of the file.
