The XDND specification can be found here:
https://www.freedesktop.org/wiki/Specifications/XDND/
and states that
| All data types are referred to by their corresponding X Atoms. The | atom names are the corresponding MIME types, in all lower case. (RFC's | for MIME: 2045, 2047, 2048, 2049) | | For text, the charset attribute can be appended to the MIME | type. (e.g. Japanese -> text/plain;charset="ISO-2022-JP") If the | charset attribute is not specified, it is assumed to be | ISO-8859-1. (new in version 4)
STRING is one of the valid _clipboard_ types (see https://tronche.com/gui/x/icccm/sec-2.html#s-2.6.2), which might be the source of the confusion?