Returns a changeset for the given struct (see Ecto.Changeset.cast/4).

Casts file uploads (Plug.Upload structs) into a generated UUID filename or, if the :cast option is present, into a value obtained by a call to the user-defined cast function.

In order not to lose the Plug.Upload structs after casting, these structs are stored into virtual fields named by the original fields that held the file uploads prefixed by "uploaded_".

Example:

• A param named "image" contains a file upload (Plug.Upload struct);
• cast_with_upload/3 casts the Plug.Upload struct into a filename;
• the changeset field :image contains the filename;
• the changeset field :uploaded_image contains the Plug.Upload struct.