Hey @Tiff,
Trial accounts restrict which file types can be uploaded. Supported formats are GIF, JPEG, PNG, WebP, HEIC, PDF, CSV, JSON, and JS/CSS. Anything outside that list (video, 3D models, etc.) requires a paid plan: https://help.shopify.com/en/manual/shopify-admin/productivity-tools/file-uploads
If you are using a supported file, another issue we have seen was around the URL format. In the post below, they were using a proxy service where the base URL before the query string didn’t end in .png or .jpg, so Shopify couldn’t identify the file type and rejected it on trial accounts. Adding the extension to the base URL fixed it: Issue Uploading Image via GraphQL API on Trial Store