MediumBlob in Laravel database schema

You can't.

This issue suggests using raw queries to create such columns, so you should do something like this in your migration file :

Schema::create("<table name>", function($table) {
    // here you do all columns supported by the schema builder
});

// once the table is created use a raw query to ALTER it and add the MEDIUMBLOB
DB::statement("ALTER TABLE <table name> ADD <column name> MEDIUMBLOB");