download attachments from mail using microsoft graph rest api

For attachment resource of file type contentBytes property returns

base64-encoded contents of the file

Example

The following Node.js example demonstrates how to get attachment properties along with attachment content (there is a dependency to request library):

const attachment = await getAttachment(
    userId,
    mesasageId,
    attachmentId,
    accessToken
);
const fileContent = new Buffer(attachment.contentBytes, 'base64');
//...

where

const requestAsync = options => {
  return new Promise((resolve, reject) => {
    request(options, (error, res, body) => {
      if (!error && res.statusCode == 200) {
        resolve(body);
      } else {
        reject(error);
      }
    });
  });
};

const getAttachment = (userId, messageId, attachmentId, accessToken) => {
  return requestAsync({
    url: `https://graph.microsoft.com/v1.0/users/${userId}/messages/${messageId}/attachments/${attachmentId}`,
    method: "GET",
    headers: {
      Authorization: `Bearer ${accessToken}`,
      Accept: "application/json;odata.metadata=none"
    }
  }).then(data => {
    return JSON.parse(data);
  });
};

Update

The following example demonstrates how to download attachment as a file in a browser

try {
  const attachment = await getAttachment(
    userId,
    mesasageId,
    attachmentId,
    accessToken
  );

  download("data:application/pdf;base64," +  attachment.contentBytes, "Sample.pdf","application/pdf");
} catch (ex) {
  console.log(ex);
}

where

async function getAttachment(userId, messageId, attachmentId, accessToken){
    const res = await fetch(
      `https://graph.microsoft.com/v1.0/users/${userId}/messages/${messageId}/attachments/${attachmentId}`,
      {
        method: "GET",
        headers: {
          Authorization: `Bearer ${accessToken}`,
          Accept: "application/json;odata.metadata=none"
        }
      }
    );
    return res.json();
 }

Dependency: download.js library