File user properties capture the value of a file values sent in a track event, as a base 64 encoded string. These values can then be used to send email attachments.
File User Property
For example, one might create a File type user property to capture to send receipt’s for a purchase. Then, this file will be assigned as a user property value, which is accessible in templates. 
{
  "type": "track",
  "userId": "1234",
  "messageId": "72b988f9-97f9-42dd-aeac-75e154cf50e8",
  "event": "purchase",
  "files": [
    {
      "type": "Base64Encoded",
      "name": "receipt.pdf",
      "mimeType": "application/pdf",
      "data": "base64 encoded string"
    }
  ],
  "properties": {
    "orderId": "123456"
  }
}
Like Performed user properties, File user properties are calculated synchronously from within event entry Journeys. This allows you to include files in the same track event used to trigger journeys, and guarantee that the file is used to render message templates for those journeys.

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Example Use Case - Sending an Email Attachment

See the Email Editor section for a full example of how to send an email attachment.

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Walkthrough Video