Allows you to upload large files to SimpleProof for verification and timestamping on the Bitcoin blockchain.

Endpoint: POST /api/proof/file
Description: This endpoint enables you to upload large files for verification. It uses multipart/form-data to efficiently handle the uploading of large files.

Content-Type: multipart/form-data
Form Parameters:
- files (files, required): The file or files you wish to upload. It should be provided in binary format.
- callback_url (string, optional): Valid URL to receive a POST request with the result in JSON format.

200 OK

The request was successful, and the file was received and processed correctly.
```
{
  "ok": true,
  "files":[
    "FILE_HASH"
  ]
}
```
- ok (boolean): Indicates if the operation was successful.
- hash (string): The SHA-256 hash of the uploaded file.
400 Bad Request

The request was invalid or there was an error processing the file.
```
{
  "ok": false,
  "msg": "Something went wrong."
}
```
- ok (boolean): Indicates that the operation failed.
- msg (string): Descriptive error message.