Make sure you name everything the exact same thing as what I have if you are copying and pasting from here. You will need to add in your organization name.

The schema is:
{
"type": "object",
"properties": {
"Id": {
"type": "string"
},
"Name": {
"type": "string"
},
"ContentBytes": {
"type": "string"
},
"ContentType": {
"type": "string"
},
"Size": {
"type": "integer"
}
}
}
Then
the binary function is:
binary(body('Parse_JSON')?['ContentBytes'])
Make sure you change the areaPath in the relative URI. I left it as mine which is just Portal.
Then
The 'Body' in 'Compose' is the body that you will get from the Http Post which I have named 'Post Attachment'
The 'url' is actually an output function:
outputs('Compose')['url']
The outputs function is getting the url property from the response json from Post Attachment.
Then
The 'Id' is the work Item Id that gets returned form the create a new work item step.
The 'Outputs' is from 'Compose2'. It should pop up for you on the add dynamic content popup.