Below is the URL the Web Application use to authenticate with Salesforce and get the Auth Code for further API calls.
https://login.salesforce.com/services/oauth2/authorize?response_type=code&client_id=
<client_id>&redirect_uri=<callback_uri>&state=<state>

To get the client Id we need to create a Connected App in Salesforce and use the details to connect to Salesforce. This is clear.

But, how does the applications like Aside.io, Workbench etc., connect to any salesforce Org? What client Id these applications pass to different salesforce orgs for authentication? Can anyone explain it?


 
 

Code Details:

QueryResult queryResults = partnerConnection.query("SELECT Body FROM Attachment WHERE Id = '"+attachment_id+"'");    
SObject[] sObjList = queryResults.getRecords();
SObject attachment = sObjList[0];

Now I need to store the attachment (JPG file) as a file in my Server. Please provide your suggestions to achieve it. Thanks. 
            

Code Details:

QueryResult queryResults = partnerConnection.query("SELECT Body FROM Attachment WHERE Id = '"+attachment_id+"'");    
SObject[] sObjList = queryResults.getRecords();
SObject attachment = sObjList[0];

Now I need to store the attachment (JPG file) as a file in my Server. Please provide your suggestions to achieve it. Thanks.