php保存的文件下载不是预期的

I have MySQL table with LONGBLOB column to save file (although I know saving files directly in database is not a good practice.) Saving works fine! concluding (C:\ProgramData\MySQL\MySQL Server 5.7\Data\dogsport) size of dogsport has increased to the new file saved.

I have written download script part. File downloads to the computer fine (not displaying image in the webpage).

Problem: Downloaded file shows only 4 bytes although it should be 4.83 MB. When I open the image, image is not shown also (see screenshot)

What is the reason?

Updated code for @Andrew:

            $query = "SELECT Id,Name,Type,SizeMB FROM Upload"; // Good practice. Do not process much data here. leave Content field
        $result = mysqli_query ( $con, $query) or die ("Couldn't execute SELECT query: ". mysqli_error($con));
        $nrows = mysqli_num_rows($result);
        echo '<table>
                    <tr>
                        <td>Id</td>
                        <td>Name</td>
                        <td>Type</td>
                        <td>Size (MB)</td>
                    </tr>';
        $selfpg = $_SERVER['PHP_SELF'];
        while( $row = mysqli_fetch_assoc($result) ){
            extract($row);
            echo "<tr>
                        <td>$Id</td>
                        <td>$Name</td>
                        <td>$Type</td>
                        <td>$SizeMB</td>
                        <td> <a href='$selfpg?id=$Id'> Download </a> </td>
                  </tr>";       
        }
        echo '</table>';

        //----------------------------------------------------------------------
        if ( isset($_GET['id']) ){

            $result_id = "SELECT Content FROM Upload WHERE Id=". $_GET['id'];

            $row = mysqli_fetch_assoc($result_id);
            extract($row);
            $size_bytes = $SizeMB * 1024 * 1024;
            header("Content-Type: ". $Type);
            header("Content-Length: ". $size_bytes);
            header("Content-Disposition: attachment; filename= ". $Name);

            echo $Content;
        }
        else{
            echo "<br>No id received to download a file</br>";
        }

Still there is the problem

UPDATE !!! - Please download my code with SQL

http://s000.tinyupload.com/index.php?file_id=88155436688298142745

Your problem can be caused by this

header("Content-Length: ". $SizeMB);

Content-Length header must contain size in bytes, not megabytes.

Also, SELECT * FROM Upload - this is bad practice. Your query will return all fields from table, including longblob column. You don't need to process so much data. In script you are using only Id, Name, Type and SizeMB, so select only those fields. And instead

mysqli_data_seek($result,0);

you can perform second query:

SELECT Content FROM Upload WHERE Id = $Id