Also, just in case @j-bromp or another user comes across this thread and decides they also want to use this attach array in something like a Teams approval, you can't. You need a second attach array for that.
Emails (e.g., Send and Email V2) uses ContentBytes.
[
{
"Name": "My name",
"ContentBytes": "My content"
}
]
Teams Approvals uses content
[
{
"name": "My name",
"content": "My content"
}
]
And one last thing. You should always assume that things like this are case sensitive. So please also include the case as it appears here.