Updated for iOS

Not working in iPhone due to presence of charset utf-8

ios/RNFetchBlobReqBuilder.m

@@ -69,7 +69,7 @@
                 [mheaders setValue:[NSString stringWithFormat:@"%lu",[postData length]] forKey:@"Content-Length"];
                 [mheaders setValue:@"100-continue" forKey:@"Expect"];
                 // appaned boundary to content-type
-                [mheaders setValue:[NSString stringWithFormat:@"multipart/form-data; charset=utf-8; boundary=%@", boundary] forKey:@"content-type"];
+                [mheaders setValue:[NSString stringWithFormat:@"multipart/form-data; boundary=%@", boundary] forKey:@"content-type"];
                 [request setHTTPMethod: method];
                 [request setAllHTTPHeaderFields:mheaders];
                 onComplete(request, [formData length]);