MultipartEncoder.h

#import <Foundation/Foundation.h>

@interface MultipartEncoder : NSObject

- (id)initWithURL:(NSString*)urlString;

-(void) addField:(NSString*)name value:(NSString*)value;
-(void) addFile:(NSString*)name data:(NSData*)fileData;
-(void) finish;

@property (nonatomic, strong) NSMutableData* data;
@property (nonatomic, strong) NSString *boundary;
@property (nonatomic, strong) NSMutableURLRequest *request;


@end

MultipartEncoder.m

#import "MultipartEncoder.h"

@implementation MultipartEncoder

@synthesize request,boundary,data;

- (id)initWithURL:(NSString*)urlString
{
    self = [super init];
    if (self) {
        self.boundary = @"hji0s3hrgnsai";
        self.data = [NSMutableData new];
        self.request = [NSMutableURLRequest requestWithURL:[NSURL URLWithString:urlString]];
        [self.request setHTTPMethod:@"POST"];
        NSString *contentType = [NSString stringWithFormat:@"multipart/form-data; boundary=%@", self.boundary];
        [self.request addValue:contentType forHTTPHeaderField: @"Content-Type"];
    }
    return self;
}

-(void) addField:(NSString*)name value:(NSString*)value {
    NSString *header = [NSString stringWithFormat:@"Content-Disposition: form-data; name=\"%@\"", name];
    [self addPart:[value dataUsingEncoding:NSUTF8StringEncoding] headers:@[header]];
}

-(void) addFile:(NSString*)name data:(NSData*)fileData {
    NSString *fileName = @"test.jpg";
    NSString *fieldHeader = [NSString stringWithFormat:@"Content-Disposition: form-data; name=\"%@\"; filename=\"%@\"", name, fileName];
    NSString *fileHeader = @"Content-Type: application/octet-stream";
    [self addPart:fileData headers:@[fieldHeader, fileHeader]];
}

-(void) addPart:(NSData*)part headers:(NSArray*)headers {
    [self.data appendData:[[NSString stringWithFormat:@"\r\n--%@\r\n", self.boundary] dataUsingEncoding:NSUTF8StringEncoding]];
    for (NSString* header in headers) {
        [self.data appendData:[[header stringByAppendingString:@"\r\n"] dataUsingEncoding:NSUTF8StringEncoding]];
    }
    [self.data appendData:[@"\r\n" dataUsingEncoding:NSUTF8StringEncoding]];
    [self.data appendData:[NSData dataWithData:part]];
}

-(void) finish {
    [self.data appendData:[[NSString stringWithFormat:@"\r\n--%@--\r\n", self.boundary] dataUsingEncoding:NSUTF8StringEncoding]];
    [self.request setHTTPBody:self.data];
}

@end