通过Android在PHP中存储base64编码图像的安全措施

I have an api which accepts image in base64 encoded format from an android app, stores in in a location. The code that I found online for doing so is :

// Get image string posted from Android App
    $base=$_REQUEST['image'];
    // Get file name posted from Android App
    $filename = $_REQUEST['filename'];
    // Decode Image
    $binary=base64_decode($base);
    header('Content-Type: bitmap; charset=utf-8');
    // Images will be saved under 'www/imgupload/uplodedimages' folder
    $file = fopen('uploadedimages/'.$filename, 'wb');
    // Create File
    fwrite($file, $binary);
    fclose($file);

• I am a bit concerned about the security of the content that is coming. In case of multipart form data I had lots of methods to check the image. Don't know what measures to take in this case. Is this secure enough ?
• Also can the image be posted in multipart form data from android to this api ?
• Or can I convert it to form data at the server end ?

I had my own, maybe not so popular way! I add some special string in beginning of the base 64 string, and in php, I check for it, if it was present, I know it was from my app and I remove the string from the base64 and decode and save the image! The php code, is it me thing like this : Here $base is the base 64 string came from app, which should have the prefix!

// here, $prefix, is the string that we added in the beginning of our base64 string 
$prefix = "*%%*SOME_SPECIAL_CHARACTERS*%%*"; 
if (substr($base, 0, strlen($prefix)) == $prefix) { 
   $base = substr($base, strlen($prefix)); 
} else { 
   networkError("CHAR", -2); 
} 
// Decode Image 
$binary = base64_decode($base);

Of curse still a professional hacker can hack this, but they hack anything