Try example from RT-20298.
The String inserted into clipboard (with custom data format) is transformed into a byte buffer in quantum clipboard:
Not sure if this is intended, but if it's not and we want to put a string into a clipboard in such case (not serialized java.lang.String), then the following code from QuantumClipboard is causing the issue:

if ((dataFormat != DataFormat.PLAIN_TEXT &&
                            dataFormat != DataFormat.HTML) ||
                            ! (data instanceof String)) {