Digging up further (and researching a LOT), I figured it out:
About image thumbnailers: It seems that Nautilus uses libgnome-desktop for thumbnail management, and the latter uses libgdk-pixbuf to handle file types that have no external handler in /usr/share/thumbnailers. So the "basic" image types are ultimately handled by GnomeDesktopThumbnailFactory and GdkPixbuf (references shown for versions shipped in Ubuntu 12.04). And the formats it handles can be inspected using the gdk-pixbuf-query-loaders tool. In my system this means:
The GdkPixdata format
['gdkp']
['image/x-gdkpixdata']
The Sun raster image format
['ras']
['image/x-cmu-raster', 'image/x-sun-raster']
The PNG image format
['png']
['image/png']
The WBMP image format
['wbmp']
['image/vnd.wap.wbmp']
The JPEG image format
['jpeg', 'jpe', 'jpg']
['image/jpeg']
The ANI image format
['ani']
['application/x-navi-animation']
The TIFF image format
['tiff', 'tif']
['image/tiff']
The XBM image format
['xbm']
['image/x-xbitmap']
The PCX image format
['pcx']
['image/x-pcx']
The Targa image format
['tga', 'targa']
['image/x-tga']
The PNM/PBM/PGM/PPM image format family
['pnm', 'pbm', 'pgm', 'ppm']
['image/x-portable-anymap', 'image/x-portable-bitmap',
'image/x-portable-graymap', 'image/x-portable-pixmap']
The JPEG 2000 image format
['jp2', 'jpc', 'jpx', 'j2k', 'jpf']
['image/jp2', 'image/jpeg2000', 'image/jpx']
The ICNS image format
['icns']
['image/x-icns']
The BMP image format
['bmp']
['image/bmp', 'image/x-bmp', 'image/x-MS-bmp']
Windows Metafile
['wmf', 'apm']
['image/x-wmf']
The QTIF image format
['qtif', 'qif']
['image/x-quicktime', 'image/qtif']
Scalable Vector Graphics
['svg', 'svgz', 'svg.gz']
['image/svg+xml', 'image/svg', 'image/svg-xml', 'image/vnd.adobe.svg+xml',
'text/xml-svg', 'image/svg+xml-compressed']
The ICO image format
['ico', 'cur']
['image/x-icon', 'image/x-ico', 'image/x-win-bitmap']
The XPM image format
['xpm']
['image/x-xpixmap']
The GIF image format
['gif']
['image/gif']
So these are the "internal" thumbnailers. And it does include xpm files. So something was wrong with my particular files instead.
And indeed the files were not standard, they had a copyright note prior to the /* XPM * header:
// This file is part of BOINC.
// http://boinc.berkeley.edu
//
/* XPM */
static const char *atiicon_xpm[] = {
Once comments were removed, thumbnails were generated just file.
It is curious that even with the "invalid" header the default Image Viewer in Gnome ("Eye of The Gnome", or eog) was able to display the files! So either files are not actually invalid, or kudos to eog for being able to open them! Anyway, since it is possible to open, and eog uses gdk-pixbuf too, I've filled a bug in libgnome-desktop requesting that thumbnailers be generated for such non-standard xpm files. Any help promoting the bug is appreciated!
Also, some notes about my research:
ThumbnailerSpec is mentioned in several places as the specification for "external thumbnailers". But it has nothing to do with this! It talks about using D-Bus for thumbnailers management, and nothing indicates that such spec is actually implemented in Gnome.
Actually, I was unable to find any official spec on the implemented behavior of "saving a .thumbnailer file with [Thumbnailer Entry] and .desktop-esque format to /usr/share/thumbnailers". Bug 636819 is cited a lot as a source, but it only describes that behavior was changed in Gnome2->Gnome3 transition, and it no longer uses gconf to store external thumbnailers.
It looks like there is no spec at all. This obsolete documentation is still the only official reference, for example, on what %i %o %u %s means.
And the only "official" place announcing the change from gconf to /usr/share/thumbnailers, other than hunting down the source code commit that did it, is this seemingly ordinary bug report.
