Created attachment 288432 [details] [review]
thumbnail-look-for-preview-icon-first.patch

I think that instead of doing this in gnome-desktop, you should do this in the calling application, I'm guessing nautilus here.

I disagree - gnome-desktop already has logic that falls back to the preview icon attribute when the thumbnailer fails, like explained here.
Using it when available to bypass the external thumbnailer sounds like a policy that belongs in the library rather than in every client, since the file manager is not the only application that will read and request this.
Currently this attribute is only set by gvfs, so I think the proposed patch is ultimately reasonable; alternatively you should make the totem thumbnailer fail when it detects it's on MTP to trigger the existing fallback logic.

So what is the status here? This is still a major issue in Gnome 3.18. The gphoto2 gvfs backend is pretty much useless right now.

(I also notice that my nautilus is configured to only do thumbnails of files < 10 MiB, yet it is still trying to process my 2 GiB files)

I pushed patch for gphoto2 backend to change G_FILESYSTEM_PREVIEW_TYPE_IF_LOCAL to _NEVER (Bug 738503) and tries to convince Nautilus to use preview icons provided by the backend itself (Bug 765987).

Created attachment 330400 [details] [review]
thumbnailer: Always prefer the backend provided preview

If a preview exists for a particular file, in particular a preview icon
for videos and images on external devices, prefer those to running a
script.

Review of attachment 330400 [details] [review]:

Looks good to me, fwiw

Attachment 330400 [details] pushed as 370b985 - thumbnailer: Always prefer the backend provided preview