[SalesForce] Get Attachment which is of Type File from Rest API

You can do this using forcetk if you are planning to do this in javascript

here is sample code -

<apex:page docType="html-5.0" title="File Uploader">

Select a file to upload as a new Chatter File.

client.setSessionToken('{!$Api.Session_ID}');

function upload() {
    var file = $("#file")[0].files[0];
    client.createBlob('ContentVersion', {
        Origin: 'H', // 'H' for Chatter File, 'C' for Content Document
        PathOnClient: file.name
    }, file.name, 'VersionData', file, function(response){
        console.log(response);
        $("#message").html("Chatter File created: <a target=\"_blank\" href=\"/" + response.id + "\">Take a look!</a>");
    }, function(request, status, response){
        $("#message").html("Error: " + status);
    });
}

here is internal forcetk implementation for sf blob [link - https://github.com/developerforce/Force.com-JavaScript-REST-Toolkit/blob/master/forcetk.js] -

    /* Low level function to create/update records with blob data
 * @param path resource path relative to /services/data
 * @param fields an object containing initial field names and values for 
 *               the record, e.g. {ContentDocumentId: "069D00000000so2", 
 *               PathOnClient: "Q1 Sales Brochure.pdf"}
 * @param filename filename for blob data; e.g. "Q1 Sales Brochure.pdf"
 * @param payloadField 'VersionData' for ContentVersion, 'Body' for Document
 * @param payload Blob, File, ArrayBuffer (Typed Array), or String payload
 * @param callback function to which response will be passed
 * @param [error=null] function to which response will be passed in case of error
 * @param retry true if we've already tried refresh token flow once
 */
forcetk.Client.prototype.blob = function(path, fields, filename, payloadField, payload, callback, error, retry) { 
    var that = this;
    var url = (this.visualforce ? '' : this.instanceUrl) + '/services/data' + path;
    var boundary = randomString();

    var blob = new Blob([
        "--boundary_" + boundary + '\n' 
        + "Content-Disposition: form-data; name=\"entity_content\";" + "\n" 
        + "Content-Type: application/json" + "\n\n" 
        + JSON.stringify(fields) 
        + "\n\n" 
        + "--boundary_" + boundary + "\n" 
        + "Content-Type: application/octet-stream" + "\n" 
        + "Content-Disposition: form-data; name=\"" + payloadField 
          + "\"; filename=\"" + filename + "\"\n\n",
        payload,
        "\n\n" 
        + "--boundary_" + boundary + "--"
    ], {type : 'multipart/form-data; boundary=\"boundary_' + boundary + '\"'});

    var request = new XMLHttpRequest();
    request.open("POST", (this.proxyUrl !== null && ! this.visualforce) ? this.proxyUrl: url, this.asyncAjax);

    request.setRequestHeader('Accept', 'application/json');
    request.setRequestHeader(this.authzHeader, "Bearer " + this.sessionId);
    request.setRequestHeader('X-User-Agent', 'salesforce-toolkit-rest-javascript/' + this.apiVersion);
    if (this.proxyUrl !== null && ! this.visualforce) {
        request.setRequestHeader('SalesforceProxy-Endpoint', url);
    }

    if (this.asyncAjax) {
        request.onreadystatechange = function() {
            // continue if the process is completed
            if (request.readyState == 4) {
                // continue only if HTTP status is good
                if (request.status >= 200 && request.status < 300) {
                    // retrieve the response
                    callback(request.response ? JSON.parse(request.response) : null);
                } else if(request.status == 401 && !retry) {
                    that.refreshAccessToken(function(oauthResponse) {
                        that.setSessionToken(oauthResponse.access_token, null,oauthResponse.instance_url);
                        that.blob(path, fields, fileName, file, callback, error, true);
                    },
                    error);
                } else {
                    // return status message
                    error(request, request.statusText, request.response);
                }
            }            
        }
    }

    request.send(blob);

    return this.asyncAjax ? null : JSON.parse(request.response);
}

This is possible in java. I tried your code and it is working. As per my observation only thing which could be culprit here is the uri. Below is the code snippet where I have mentioned uri directly.

import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;

import org.apache.commons.httpclient.HttpClient;
import org.apache.commons.httpclient.methods.PostMethod;
import org.apache.commons.httpclient.methods.StringRequestEntity;
import org.json.simple.parser.JSONParser;
import org.json.simple.parser.ParseException;


public class Main {

        public static void main(String[] args) throws FileNotFoundException, IOException, ParseException {
            JSONParser parser = new JSONParser();

               // Parser parsing  JSON File having multiple records
               Object obj = parser.parse(new FileReader("JsonData.json"));

               HttpClient httpclient = new HttpClient();
               PostMethod mypost = new PostMethod("https://kmforce-dev-ed.my.salesforce.com/services/data/v34.0/composite/tree/Account/");
               String jsonText = obj.toString();
               System.out.println("\n_JSON TEXT=" + jsonText);
               mypost.setRequestHeader("Authorization", "OAuth " +"<myssessionid>"); 
               mypost.setRequestEntity(new StringRequestEntity(jsonText, "application/json", "UTF-8"));
               int response=httpclient.executeMethod(mypost);
               System.out.println(response);
               System.out.println("Response-->"+mypost.getResponseBodyAsString());
        }
}

Below is the output:

{
    "hasErrors": false,
    "results": [
        {
            "referenceId": "ref1",
            "id": "0019000001bslYJAAY"
        },
        {
            "referenceId": "ref2",
            "id": "0019000001bslYKAAY"
        },
        {
            "referenceId": "ref3",
            "id": "0019000001bslYLAAY"
        },
        {
            "referenceId": "ref4",
            "id": "0019000001bslYMAAY"
        }
    ]
}

Let me know if you face any issue here.