Not quite sure what is going on here, some help would be appreciated.
Overview of this Flow: PowerApp saves to SharePoint List (that is working fine), and it kicks off this flow. This flow checks for attachments, if it has any, it goes through and grabs those files into an initialized array.
When it gets to the email sending, it bombs with Error 400.
I can reproduce this issue on my side. It seems that when the attachment variables are used in the flow, it would return the error message you mentioned. While without using the attachment variables, the flow would send emails with separate attachment.
Please try to use dynamic contents of Attachment name and Attachment content instead of variables.
I will help report this issue on my side and back to you later.