MySQL InnoDB AUTO_INCREMENT secondary column

Question

I am making a system where users can upload any file they want, and not use it to execute any kind of code. As a part of that, I rename every file, and store its original name in a MySQL table. This table contains the id of the user who uploaded it, and a unique id of the upload. Currently I am doing it like this:

CREATE TABLE `uploads` (
    `user_id` INT(11) NOT NULL,
    `upload_id` INT(11) NOT NULL AUTO_INCREMENT,
    `original_name` VARCHAR(30) NOT NULL,
    `mime_type` VARCHAR(30) NOT NULL,
    `name` VARCHAR(50) NOT NULL,
    PRIMARY KEY (`user_id`, `upload_id`)
) ENGINE=MyISAM;

This means I will always have a unique combination of user_id and upload_id, and every users first upload has an id of 1. However I want to use a foreign key for the user_Id, so if I delete a user, its uploads would also be deleted. This means I have to do it in InnoDB. How would i go about that, since the above setup only works in MyISAM.

My users table (wich i would get user_id from) looks like this:

    CREATE TABLE `".DATABASE."`.`users` (
    `user_id` INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
    `username` VARCHAR(30) NOT NULL,
    `email` VARCHAR(50) NOT NULL,
    `password` CHAR(128) NOT NULL,
    `salt` CHAR(128) NOT NULL 
) ENGINE = InnoDB;

What i want is for the uploads table to look like this:

user_id  | upload_id
1        | 1
1        | 2
2        | 1
2        | 2
2        | 3
1        | 3

If that makes sense

Answer 1

If I understood correctly:

Replace the primary key to a unique index with the two fields. Make the upload_id the primary key and user_id the foreign key then.

Answer 2

Unfortunately for this problem, the auto increment column will not start over at 1 for each user. It will just increment for each added row, regardless of any particular column value.

Edit: displaying my ignorance there, MyISAM tables apparently will start over at 1 for each user when a multi-column index this way. https://dev.mysql.com/doc/refman/5.0/en/example-auto-increment.html

Fortunately, it is probably not necessary for upload_id to start at 1 for each user. Using auto increment on that column means you will always have a unique combination of user_id and upload_id even without using both columns as the primary key, or even creating a unique index, because every record will have a different upload_id . You should still be able to implement the cascade delete with this setup.