quick hack to support custom content-types

src/ios/RNFetchBlobReqBuilder.m

         {
             NSString * name = [field valueForKey:@"name"];
             NSString * content = [field valueForKey:@"data"];
+            NSString * contentType = [field valueForKey:@"type"];
             // field is a text field
             if([field valueForKey:@"filename"] == nil || content == [NSNull null]) {
                 [formData appendData:[[NSString stringWithFormat:@"--%@\r\n", boundary] dataUsingEncoding:NSUTF8StringEncoding]];
                             NSString * filename = [field valueForKey:@"filename"];
                             [formData appendData:[[NSString stringWithFormat:@"--%@\r\n", boundary] dataUsingEncoding:NSUTF8StringEncoding]];
                             [formData appendData:[[NSString stringWithFormat:@"Content-Disposition: form-data; name=\"%@\"; filename=\"%@\"\r\n", name, filename] dataUsingEncoding:NSUTF8StringEncoding]];
-                            [formData appendData:[[NSString stringWithFormat:@"Content-Type: application/octet-stream\r\n\r\n"] dataUsingEncoding:NSUTF8StringEncoding]];
+                            [formData appendData:[[NSString stringWithFormat:@"Content-Type: %@\r\n\r\n", contentType] dataUsingEncoding:NSUTF8StringEncoding]];
                             [formData appendData:content];
                             [formData appendData:[[NSString stringWithFormat:@"\r\n"] dataUsingEncoding:NSUTF8StringEncoding]];
                             i++;
                 NSString * filename = [field valueForKey:@"filename"];
                 [formData appendData:[[NSString stringWithFormat:@"--%@\r\n", boundary] dataUsingEncoding:NSUTF8StringEncoding]];
                 [formData appendData:[[NSString stringWithFormat:@"Content-Disposition: form-data; name=\"%@\"; filename=\"%@\"\r\n", name, filename] dataUsingEncoding:NSUTF8StringEncoding]];
-                [formData appendData:[[NSString stringWithFormat:@"Content-Type: application/octet-stream\r\n\r\n"] dataUsingEncoding:NSUTF8StringEncoding]];
+                [formData appendData:[[NSString stringWithFormat:@"Content-Type: %@\r\n\r\n", contentType] dataUsingEncoding:NSUTF8StringEncoding]];
                 [formData appendData:blobData];
                 [formData appendData:[[NSString stringWithFormat:@"\r\n"] dataUsingEncoding:NSUTF8StringEncoding]];
             }
 }
 
 
-@end
+@end