You can attach any files you need to your challenge. Typically these include input data for the algorithms, additional resources and descriptions, example solutions etc. There are no restrictions regarding their format nor contents. They are stored in the challenge folder in Repository and can be uploaded via the Repository page.
There are two special files that need to be attached to a challenge before it is opened: the preliminary and final test datasets. They are used to evaluate participants' solutions. Unlike other files, they must be uploaded via "Data files" page. In order to do that, you can prepare those files yourself and choose them in the "Test datasets" section. Alternatively, you can also use the Data Wizard tool which will generate them automatically.
The required format of test data depends heavily on the used evaluation procedure. Here you can find the details about the data format for the standard evaluation procedures. For non-standard ones, please refer to the respective procedure's description.