Ernst says that "dosser = to fold, to ply [sic!]". I therefore suspect a "film dossé" may be of a width greater than the roll itself, i.e. it is folded inwards, over itself, in order to put a greater width on a narrower roll. I have such a roll of plastic that I "inherited" when someone retired: the roll is about 1m wide, but if you cut off a piece of the film and unfold it, it is about 3 m wide.

In any case, "dossé" is often juxtaposed to "à plat":
See picture of "gaine à plat" and "peut être aussi dossé" at:
www.gdcemballages.com/index.php?id_p=6

The BECK-MULTIPLEX is especially interesting for contract packers with a working
width ranging from 20-400 mm and a maximum height of 200 mm. The machine can be equipped with a wide range of special functions. These options include: suspension packs with sealed Euro-slot, perforations for an easy opening of the packs and many other possibilities for opening and closing the pack as well as the possibility of working printed film with register print and the alternate processing of CENTRE-FOLDED or flat films.
www.beck-packautomaten.de/English/d-news.htm
La BECK-MULTIPLEX, avec sa largeur de travail de 20 à 400 mm et son hauteur de passage jusqu'à 200 mm, est particulièrement intéressante pour les conditionneurs à façon. De plus, elle peut être équipée d'options spéciales, pour réaliser par exemple des sachets de suspension avec trou européen soudé, des prédécoupes pour une ouverture facile des paquets, différentes autres options d'ouverture et de fermeture ainsi que la possibilité d'utiliser des films avec impression spotée, ou de travailler soit avec du film DOSSE ou à plat
www.beck-packautomaten.de/Franz/d-news.htm

Centre-folded is exactly what my roll of film is, and what can be seen in the gdcemballages site above. By folding one side two-thirds of the way across the width, then folding the top layer back to the edge, then folding the other side over the top of what has been folded previously, then back to its own edge, you effectively get three times the width of film on a roll.

I'd give higher confidence if it weren't for the "trancannée"; though I suppose there is no reason why a centre-folded film should not be cross-wound, I don't see why it should!

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Note added at 2 hrs (2007-05-01 23:14:41 GMT)
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centre folded polyolefin film. 30 micron. 30 micron. 30 micron. 30 micron ...
www.wrapsuk.com/downloads/Wraps_UK_W700_L_sealer.pdf

PVC Shrink packaging film supplied centre folded is mainly used in conjunction with manually operated 'L' sealers giving a glossy, clear, tamperproof finish ...
www.wrapsuk.com/shrink-wrap-film.htm

Available in flat and centre folded film, with or without perforation. Available thicknesses: 12,5µ, 15µ, 19µ, 25µ. (50, 60, 75, 100 gauge). ...
packexpo.com/ve/36320/main.html

The units are designed to create a centre folded film from a flat film and especially suited for the KALLFASS Fully-Automatic L-Sealer and Side-Sealer ...
www.kallfass.com/e_kallfass1/e_komponenten.html

But my personal favourite is another kind of centrefold ;-)