Data type request for X selection.
The value is one of the following data types, a list of them, or nil:
`COMPOUND_TEXT', `UTF8_STRING', `STRING', `TEXT'
If the value is one of the above symbols, try only the specified type.
If the value is a list of them, try each of them in the specified
order until succeed.
The value nil is the same as the list (UTF8_STRING COMPOUND_TEXT STRING).