A much simpler solution is just wrapping the setColumnType() call within a hasColumn() check, which won't require any additional changes.

Just saw your suggestion, I'll try that

I fact, I don't think so. Those fields are "virtual". They don't reside in the table. hasColum will always return false

Yes, it would and hence nothing needs to be done. In 5.0 the setColumnType() call simply returned without doing anything if the column did not actually exist in the schema. So things remain the same.

In 5.0, it would add the non-existant column in the schema, or I did something wrong

object(Cake\Database\Schema\TableSchema) id:0 {
'table' => 'attachments'
'columns' => [
    'id' => [ ],
    'name' => [ ],
    'size' => [ ],
    'type' => [ ],
    'dir' => [ ],
    'created' => [ ],
    'modified' => [ ],
    'email_heading' => [ ],
    'thumbnail' => [ ],
    'poster' => [ ],
    'illustration' => [ ],
]

email_heading, thumbnail, poster and illustration here above are the attachments defined

In 5.0, it would add the non-existant column in the schema...

No it doesn't https://github.com/cakephp/cakephp/blob/5.0.11/src/Database/Schema/TableSchema.php#L379-L383

ah yes, sorry, I didn't clear the cache after having downgraded to 5.0.

Instead of changing this add the field column to the schema.

Do you mean adding the field column in tests/schema.php? Doing this does nothing by itself, and test are failing:

1. Josegonzalez\Upload\Test\TestCase\Model\Behavior\UploadBehaviorTest::testInitialize
   Expectation failed for method name is "setColumnType" when invoked 1 time.
   Method was expected to be called 1 time, actually called 0 times.

What I understand is that, as the schema is a mock, I have to mock the 'hasColumn' method as well, and return true.

$schema = $this->getMockBuilder('Cake\Database\Schema\TableSchema')
   ->onlyMethods(['setColumnType', 'hasColumn'])
   ->disableOriginalConstructor()
   ->getMock();
$schema->expects($this->once())
   ->method('hasColumn')
   ->with('field')
   ->willReturn(true);

The test then passes, but then I don't see the point of modifying the schema in that case, or is there something else that I'm missing?

Yes you'll need to add expects for hasColum() too.

We need to separately test for the two cases where the column is in the schema and when it's not and not use any() as it's less work :)