Hi @tiffany
The error doesn't indicate it, but I believe this is a syntax error in the file name - it's expecting a string value. I copied your attachment string (hopefully I copied everything correctly, but double check the names...) and just added quotes around the name portion:
[{data: {{fileInput2.value[0].base64Data}}, name: 'Manifest-{{manifests_admin_table.selectedRow?.mtn}}.pdf', contentType: {{fileInput2.value[0].type}} }]