You will probably find your solution in this StackOverflow comment

Basically, you turn your column type in a blob or longblob in your TypeORM annotation, and then use the type Buffer in your Entity's field

@Column({type: 'longblob'})
attachFile: Buffer;

Then you will be able to serve the file as showed in the post example:

app.get("/file/:id", async (req, res)=>{
    try {
        const repo = getConnection().getRepository(MYFile)
        const result_find = await repo.findOne(req.params.id)
        console.log(result_find);
        var fileData = result_find.data;
        res.writeHead(200, {
        'Content-Type': result_find.mimeType,
        'Content-Disposition': 'attachment; filename=' + result_find.name,
        'Content-Length': fileData.length
        });
        res.write(fileData);
        res.end();
    } catch (error) {
        console.log(error)
        res.send("ERROR")
    }
})

if you want using string, client must send base64 file to backend.
format: data:(mimetype);(charset),(encoded) -> data:image/png;base64,\ee\9f920d....

here solution, using base64 string
DTO (data transfer object)

import { IsDefined, IsNotEmpty } from 'class-validator';

export class UpdateUserAvatarDto {
  @IsDefined()
  @IsNotEmpty()
  file: string;
}

controller

@UseGuards(JwtAuthGuard)
@Patch('account/avatar')
async updateAvatar(
    @User() user: Payload,
    @Body() updateUserAvatarDto: UpdateUserAvatarDto,
    @Res() res: Response,
  ) {
    try {
      const { file } = updateUserAvatarDto;

      createFile(file, { prefix: ['user', 'avatar'], name: user.id }); // file is string base64 you can store it to database.

      return response(res, HttpStatus.OK, {
        message: 'Successfully update avatar',
      });
    } catch (e) {
      console.error(e);
      return response(res, HttpStatus.INTERNAL_SERVER_ERROR, {
        message: e,
        data: null,
      });
    }
}

if you want to create a file from base64

export const createFile = async (base64, { prefix, name }) => {
  const cleanBase64 = base64.split(',')[1];
  const buffer = Buffer.from(cleanBase64, 'base64');
  const file = await fileType.fromBuffer(buffer);
  return fs.writeFileSync(
    path.join(
      path.resolve('./'),
      ...['public', ...prefix, `${name}.${file.ext}`],
    ),
    buffer,
  );
};