The list of item properties available as search conditions is contained in https://support.office.com/en-us/article/keyword-queries-and-search-conditions-for-content-search-c4639c2e-7223-4302-8e0d-b6e10f1c3be3?ui=en-US&rs=en-US&ad=US. Not all of these conditions appear when you use the "Add conditions" button during search query building. For example, HasAttachment and Importance are not among the 19 conditions to choose from. It is possible to add "HasAttachment:true" as a keyword, but it is not intuitive that you would add a condition to the keyword box.