dt_imageio_large_thumbnail misreports JPEG SubIFD as image/tiff on Leica Q2 DNGs

Is there an existing issue for this?

• I checked and did not find my issue in the already reported ones

Describe the bug

darktable_leica_q2_thumbnail_bug.md

report generated with help of Claude AI

Information for package darktable:

Repository : Main Repository (OSS)
Name : darktable
Version : 5.4.1-1.5
Arch : x86_64
Vendor : openSUSE
Installed Size : 33.8 MiB
Installed : Yes
Status : up-to-date
Source package : darktable-5.4.1-1.5.src
Upstream URL : http://www.darktable.org/

Steps to reproduce

darktable_leica_q2_thumbnail_bug2.md

Expected behavior

No response

Logfile | Screenshot | Screencast

No response

Commit

No response

Where did you obtain darktable from?

distro packaging

darktable version

5.4.1-1.5

What OS are you using?

Linux

What is the version of your OS?

openSUSE Tumbleweed 20260617

Describe your system

System:
Host: localhost.localdomain Kernel: 7.0.12-1-default arch: x86_64 bits: 64
Desktop: MATE v: 1.28.2 Distro: openSUSE Tumbleweed 20260617
Machine:
Type: Desktop Mobo: Micro-Star model: MAG X870 TOMAHAWK WIFI (MS-7E51)
v: 1.0 serial: Firmware: UEFI vendor: American
Megatrends LLC. v: 1.A91 date: 06/04/2026
CPU:
Info: 8-core model: AMD Ryzen 7 9700X bits: 64 type: MT MCP cache: L2: 8 MiB
Speed (MHz): avg: 605 min/max: 605/5582 cores: 1: 605 2: 605 3: 605 4: 605
5: 605 6: 605 7: 605 8: 605 9: 605 10: 605 11: 605 12: 605 13: 605 14: 605
15: 605 16: 605
Graphics:
Device-1: Advanced Micro Devices [AMD/ATI] Navi 23 [Radeon RX 6600/6600
XT/6600M] driver: amdgpu v: kernel
Device-2: Advanced Micro Devices [AMD/ATI] Granite Ridge [Radeon Graphics]
driver: amdgpu v: kernel
Display: x11 server: X.org v: 1.21.1.21 driver: X: loaded: modesetting
unloaded: vesa dri: radeonsi gpu: amdgpu resolution: 1920x1080~60Hz
API: EGL v: 1.5 drivers: radeonsi,swrast
platforms: gbm,x11,surfaceless,device
API: OpenGL v: 4.6 vendor: amd mesa v: 26.1.2 renderer: AMD Radeon RX
6600 (radeonsi navi23 ACO DRM 3.64 7.0.12-1-default)
Info: Tools: api: clinfo, eglinfo, glxinfo x11: xprop,xrandr

Are you using OpenCL GPU in darktable?

Yes

If yes, what is the GPU card and driver?

Graphics: Device-1: Advanced Micro Devices [AMD/ATI] Navi 23 [Radeon RX 6600/6600 XT/6600M] driver: amdgpu v: kernel Device-2: Advanced Micro Devices [AMD/ATI] Granite Ridge [Radeon Graphics] driver: amdgpu v: kernel

Please provide additional context if applicable. You can attach files too, but might need to rename to .txt or .zip

No response

Start at the dt_imageio_large_thumbnail entry point and review the attached reproduction reports for the Leica Q2 DNG case. Trace how the JPEG SubIFD is identified and confirm that the reported thumbnail type is no longer image/tiff for the affected files.