Which fields are unavailable within filters on the document list?

The following types of gadgets are not supported by filters:

• Short text
• Long text
• Rich text
• Time
• Number
• Email
• Any of the gadgets under the Special heading

Additionally, although linked gadgets can be used for filtering - if they use the same type of data as any of the above gadgets, they will not be available. For example, if you import data that is formatted as text, it will not be able to be used within a filter on the document list.

The fields that can't be filtered on however, can be shown in the column view on the document list. Another quick way to check is whether or not you are seeing the menu icon next to a column header and the option to filter on field, like this:
 

If this option is not present, then that field won't be able to be used within a filter.