Get details on a merchant onboarding file
{- "fileId": "lfile_Z6amK_iVnmGJghHV",
- "merchantId": "mcht_Z6amKviVnmGJghHH",
- "category": "MerchantOnboarding",
- "docType": "CompanyVerificationDocumentBack",
- "fileName": "string",
- "fileType": "jpeg",
- "fileSizeInMB": 0,
- "fileSizeInKB": 0,
- "createdAt": "2019-08-24T14:15:22Z"
}