Is the thumbnail property read/write? Or only read?

Question

I am using the Content Builder API to create assets. Specifically content blocks and I want to set the thumbnail attribute. However it appears to be read only. When I specify a value it never saves the value. I have tried setting the thumbnailUrl property to both a URL to an image I want to use as the thumbnail. And to a relative path. The relative path is what is returned when I create the asset or retrieve it. The relative path looks like this: /v1/assets/######/thumbnail. Where ###### is a the content id which is accessible in the image properties via the UI or when doing a retrieve of the asset.

Being able to reset a thumbnail would provide a richer user experience as I could create and display a graphic that approximates what my ampscript does. Rather than letting SFMC display a snippet of ampscript which is useless to marketers building the email.

I have tried using the superContent (and design) property. However the superContent property allows you to use one image. Salesforce forces that image to do "double duty" as both the thumbnail image and the content that is displayed when the content block is dragged into an email. The image used when dragging a content block into an email matches whatever size you upload. So this is fine. The issue is that salesforce uses the same image and if it exceeds (based on my testing) 300 X 200 it will crop the image. If it is less than 200 say around 75 -100 it does not fill out the thumbnail.

Again my experimentation showed a 300 pixel wide by 200 pixel tall image got the best of both worlds.

What I'm asking is there a way to set the thumbnail separate to the superContent? So I can have 2 different images for thumbnail and display? The thumbnail property is their but I can seem to write to it.

Support sent me here. I'll jump into my Delorean, turn on the gigawatts, and check your answers in the future.

Answer 1

The thumbnail property is currently read only, automatically generated from your content following https://developer.salesforce.com/docs/atlas.en-us.mc-apis.meta/mc-apis/compiling-asset-model.htm.

Your experimentation is on point. There is currently no way to write to thumbnail specifically or override it aside from the superContent.

Answer 2

First off, the thumbnail property in the asset API is read only and is auto-generated by SFMC.

So I am assuming this is concerned more around displaying emails rendered with dummy content and not in reference to content blocks or images or other asset types.

If so, you can generate a preview of your email via the endpoint below:

POST /guide/v1/emails/{{AssetID}}/dataExtension/{{yourDEObjID}}/row/{{rowNumberToUse}}/preview
Host: https://{{yourSubdomain}}.rest.marketingcloudapis.com
Authorization: Bearer {{authToken}}
Content-Type: application/json

You can also do a preview based on List and/or contacts.

emails/{id}/contacts/{cid}/preview

emails/{id}/lists/{lid}/preview

or both:

emails/{id}/lists/{lid}/contacts/{cid}/preview

You can then either take this returned content (html or text content) and run it through a converter (can have server-side code on a page to translate the return from the API to display image or run the return through an outside service, etc.) to change HTML to PNG or whatever image type you want. OR if you wanna be fancy, you can use SVG foreignObject with iFrame to display the html as an image in your site (this is what SFMC does for their previews inside of content builder.).