# FileUploadBlockHeader

Location: [gRPC API](https://developer.m-files.com/APIs/gRPC-API/index.md) > [Reference](https://developer.m-files.com/APIs/gRPC-API/Reference/index.md) > [Messages](https://developer.m-files.com/APIs/gRPC-API/Reference/Messages/index.md)

HTML version: https://developer.m-files.com/APIs/gRPC-API/Reference/Messages/MFiles.FileUploadBlockHeader/

The first message of the FileUploadBlock request stream.

- **Full name:** `MFiles.FileUploadBlockHeader`
- **Proto file:** [`mfiles/IRPCFileTransferStreaming.proto`](https://developer.m-files.com/APIs/gRPC-API/downloads/proto/mfiles/IRPCFileTransferStreaming.proto)

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## Fields

| Field | Type | Number | Description |
|---|---|---|---|
| <a id="f-file_ref"></a>`file_ref` | `string` | 1 | The file_ref received from FileUploadBegin. |
| <a id="f-block_index"></a>`block_index` | `int64` | 2 | The zero-based index of the block within the file, in content order. Must be less than 50 000. |
| <a id="f-block_size"></a>`block_size` | `uint64` | 3 | The size of the block data in bytes. Required.<br>The data must be exactly this size and must not exceed the maxBlockSize returned by FileUploadBegin. |

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## Used by methods

- [IRPCFileTransferStreaming.FileUploadBlock](https://developer.m-files.com/APIs/gRPC-API/Reference/MFiles.IRPCFileTransferStreaming/FileUploadBlock/index.md): Uploads one block of a stateless file upload by streaming its data.

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## Used in fields

- [`FileUploadBlockRequest.header`](https://developer.m-files.com/APIs/gRPC-API/Reference/MFiles.IRPCFileTransferStreaming/FileUploadBlock/index.md#request): The first message of the stream. Describes the block.