PNGs downloaded from RTS Coverage by DARTS dataset in PDG won't open

[DarianGill]

Describe the bug
Downloading the Retrogressive Thaw Slumps (RTS) Coverage by DARTS via the partial download tool at any resolution as a PNG creates files that cannot be opened by preview. While downloading an error message in the progress bar saying failed to fetch meta data for darts viz and when the PNGs are double clicked after unzipping a error window appears saying: The file “0.png” could not be opened. It may be damaged or use a file format that Preview doesn’t recognize.

To Reproduce
Steps to reproduce the behavior:

1. Go to PDG
2. Turn on the first Retrogressive Thaw Slumps (RTS) Coverage by DARTS layer under the Permafrost Thaw dropdown by clicking the eye icon to its left
3. Initiate the partial download tool by clicking the download icon between the reset and help button in the upper left of the map.
4. Click the draw button and draw a polygon by clicking the map to make points and double clicking to complete the shape
5. Click the check mark next to the Retrogressive Thaw Slumps (RTS) Coverage by DARTS data layer
6. Choose any resolution
7. Select PNG as the file type
8. Click the download button in the bottom right of the download panel
9. Note progress bar error message
10. Double click the downloaded zip file in the browser or downloads in finder to unzip it
11. Navigate the nested structure to the PNGs
12. Double click the PNGsExpected behavior
    The PNG should open like it does for other data products like the IWP and CO2 balance layers.

Screenshots

Desktop:

OS: macOS 15.7.3 (24G419)
Browser: Chrome Version 145.0.7632.160 (Official Build) (arm64)
MetacatUI Version: Since at least 9b0c1c4 and almost certainly before