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In my example I know the Case ID to attach it to, as it is already creating Attachments, and it’s already uploading data using Base64 strings, and it is just a matter of swapping out
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for the Composite API endpoint.
Sizes
The documentation seems pretty clear https://developer.salesforce.com/docs/atlas.en-us.api_rest.meta/api_rest/dome_sobject_insert_update_blob.htm#inserting_a_contentversion
You can insert or update blob data using a non-multipart message, but you are limited to 50 MB of text data or 37.5 MB of base64–encoded data.
So Base64 increases the size of the content by ~33%. So that means a 27.8MB file can be uploaded.
Cool. My client’s web restrictions are way below that. Let’s do this.