Hi,
We are replacing native functionality of Attachments to upload files to "Box"external storage using Rest API, I was able to upload small files upto 5MB but when trying to upload big files I get the error in Apex code (not even going to box) 

System.StringException: String length exceeds maximum: 6000000

This is occurig when trying to decoding ( base64Decode ) from String blob before Rest API call, please check the code.

String bodyEncoded = EncodingUtil.base64Encode(fileBody);
Blob bodyBlob = null;
String last4Bytes = bodyEncoded.substring(bodyEncoded.length()-4,bodyEncoded.length());
// GW: replacement section to get rid of padding without corrupting data
if(last4Bytes.endsWith('==')) {
    last4Bytes = last4Bytes.substring(0,2) + '0K';
    bodyEncoded = bodyEncoded.substring(0,bodyEncoded.length()-4) + last4Bytes;
    String footerEncoded = EncodingUtil.base64Encode(Blob.valueOf(footer));
    bodyBlob = EncodingUtil.base64Decode(headerEncoded+bodyEncoded+footerEncoded); //Here its failing
}else if(last4Bytes.endsWith('=')){
    last4Bytes = last4Bytes.substring(0,3) + 'N';
    bodyEncoded = bodyEncoded.substring(0,bodyEncoded.length()-4) + last4Bytes;
    footer = '\n' + footer;
    String footerEncoded = EncodingUtil.base64Encode(Blob.valueOf(footer));
    bodyBlob = EncodingUtil.base64Decode(headerEncoded+bodyEncoded+footerEncoded);              
}else{
    footer = '\r\n' + footer;
    String footerEncoded = EncodingUtil.base64Encode(Blob.valueOf(footer));
    bodyBlob = EncodingUtil.base64Decode(headerEncoded+bodyEncoded+footerEncoded);  
}

Error is triggering at "bodyBlob = EncodingUtil.base64Decode(headerEncoded+bodyEncoded+footerEncoded);" this line.
Please advice how can we ignore the size of the file? We need to upload at least 10 MB.

Thanks,
Sumant K