Get details on a merchant onboarding file
{- "fileId": "lfile_aAZ-QId-_0MUyuuJ",
- "merchantId": "mcht_aAZ-P4d-_0MUyut8",
- "category": "MerchantOnboarding",
- "docType": "CompanyVerificationDocumentBack",
- "fileName": "string",
- "fileType": "jpeg",
- "fileSizeInMB": 0,
- "fileSizeInKB": 0,
- "createdAt": "2019-08-24T14:15:22Z"
}