add preview file for this item. pszPreviewFile points to local file, which must be under 1MB in size
public static bool AddItemPreviewFile(UGCUpdateHandle_t handle, string pszPreviewFile, EItemPreviewType type) { InteropHelp.TestIfAvailableClient(); using (var pszPreviewFile2 = new InteropHelp.UTF8StringHandle(pszPreviewFile)) { return NativeMethods.ISteamUGC_AddItemPreviewFile(handle, pszPreviewFile2, type); } }