Multiple thumbnail images in job file not handled properly

[dc42]

A file may contain multiple thumbnails, but PD only stores one set of details in variable 'thumbnail', and when PD receives thumbnail data in the M36 response it overwrites any previous details. So it only stores the last details received, and they may not be suitable for PD. See https://forum.duet3d.com/topic/31840/how-to-get-thumbnails-on-paneldue/16?_=1683480130659.

A fix would be for PD to parse new thumbnail data into a temporary variable, then when parsing it is complete, if it is suitable for PD and better than the values (if any) in 'thumbnail', copy it into 'thumbnail'.