Hi @irinawork and welcome to the community.
The /rest/api/3/attachment/{id} endpoint in the Jira Cloud API also does not provide the actual width and height proportions of the thumbnail image.
However, you can use the /rest/api/3/attachment/{id}/thumbnail endpoint to retrieve the thumbnail image itself, and then use an image processing library or tool to get the actual width and height of the image and calculate its proportion.
I do not know how to get the width and height of the image to figure out the proportions, but the actual calculation once you get them would be dividing the width by its height.
I hope this helps.
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