I am getting error when i am trying to upload file from salesforce.

I am integrating salesforce with Dropbox.
But when i am trying to hit the api it showing error.

Error - 

HTTP header "Dropbox-API-Arg": could not decode input as JSON

Code - 
string token = 'https://content.dropboxapi.com/2/files/upload';
HttpRequest r = new HttpRequest();
r.setEndpoint(token);
r.setHeader('Authorization','Bearer ' +accesstoken);
r.setheader('content-type','application/octet-stream');
r.setheader('Dropbox-API-Arg', JSON.stringify({\'path\': response.data.entries[i].id}));
r.setMethod('POST');
r.setTimeout(60000);*/
Attachment at = [ SELECT Name, body
FROM Attachment
LIMIT 1];
//here we input file as blob for save in DROP BOX
r.setBodyAsBlob(at.body);
Http h1 = new Http();
HttpResponse res1;
string resp1;
res1 = h1.send(r);
resp1 = res1.getBody();