Dear Jo,
Thanks for your updates and kindness.
From your first post and screenshots, the attachments are got from Microsoft Forms. For personal form, the attachment files should be saved in the Apps folder in your OneDrive for Business site.
May I know whether you get the file based on the file identifier by using the json() formula in your flow? From the screenshot of the flow, I can't get the details about how you get the file identifier from Forms. If you directly select the dynamic file upload part from the response in the flow, it will not get the specific file identifier.
You can run a flow instance again from your side to see whether some file ContentBytes from Get File Content.
For the Append Array part, you can refer to my another thread Attachments can't be opened after they are created in a flow - Microsoft Community.
If it is convenient, please upload some detailed screenshots about the Append Array part and how you get the file identifier from Forms in your flow for our reference and we'll check whether something is needed to be modified in your flow.
Note: Remove any private information before uploading the screneshots.
Welcome to share any updates at your convenience.
Thank you for your time and effort.
Sincerely
Cliff | Microsoft Community Moderator