I’ve got a Custom Picture Library definition – it encapsulates much of the same functionality I described a few days ago for Document Libraries. I built it, then ran Solution Generator against it to create the List definition and schema.

It all seemed to work alright, but when I tried uploading an image, it didn’t create my thumbnails, or the _w and _t folders the thumbnails are hidden in! This was a problem.

Well, I cracked open reflector (how did we do .NET development before it?) and dug around. Didn’t find much, except that the Upload.aspx page decides if it is for a Picture Library based on the List Template‘s Type attribute (ordinarily 109).

The Type attribute of a List Template tag says this:

Provides a unique identifier for the template. This identifier must be unique within the feature, but need not be unique across all feature definitions or site definitions.
…
This attribute corresponds to the Type attribute of the List element

Hmm. Interesting. I had changed my List Template‘s Type – I’ve been keeping all of mine unique in my solution, as I do have multiple lists based on the same out-of-box type (e.g. multiple lists based upon the ‘Custom List’ type). As I was suspicious, I changed this back to 109, and tried again. My thumbnails now worked.

I guess that this mean that the thumbnailing done during the upload process is contingent on the list’s Type being 109 still. As a consequence, I guess that a feature can only have one custom Picture Library definition (as “This identifier must be unique within the feature”).