The solution would be to add array of attachments(since you want add multiple number of attachments) like -
"attachments": [
{
"@odata.type": "#microsoft.graph.fileAttachment",
"name": "Teste de Anexo",
contentType: 'text/plain',
contentBytes: byteContentOfFile,
},
{
"@odata.type": "#microsoft.graph.fileAttachment",
"name": "File2.txt",
contentType: 'text/plain',
contentBytes: byteContentofFile2,
},
{
"@odata.type": "#microsoft.graph.fileAttachment",
"name": "File3.txt",
contentType: 'text/plain',
contentBytes: byteContentofFile3,
},
]
Hope the solution works for you !!!