Migrating from piwigo 2.4

Piwigo 2.4 came with a lot of change and did not bother to preserve legacy features ...

One of them is the way thumbnails are displayed, especially for exotic files.

With piwigo < 2.4, the directory structure had to be :

galleries
└── Videos
    ├── video1.webm
    ├── video2.ogv
    ├── thumbnail
        ├── TN-video1.jpg
        └── TN-video2.jpg

With piwigo > 2.4, the directory structure has to be :

galleries
└── Videos
    ├── video1.webm
    ├── video2.ogv
    ├── pwg_representative
        ├── video1.jpg
        └── video2.jpg