Copy & Import: Lower-case bug $(FILE_EXTENSION,,)

Is there an existing issue for this?

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

Describe the bug

When importing JPG and RAF files of the same photograph, Darktable fails to set lowercase for the file extension of the file imported in second place.

Steps to reproduce

1. Click copy & import
2. With "file naming pattern" set to: $(JOBCODE)-$(EXIF_YEAR)$(EXIF_MONTH)$(EXIF_DAY)-$(FILE_NAME:4:4).$(FILE_EXTENSION,,)
3. Select, for instance, DSCF1001.JPG; DSCF1001.RAF.
4. Imported filenames will look like: Lisbon-20260131-1001.jpg; Lisbon-20260131-1001.RAF. The Lisbon-20260131-1001.RAF should be Lisbon-20260131-1001.raf.

Note: If I import first all JPG and then all RAF, the intended lowercase pattern is fully reflected in the imported filenames.
I also tried
$(JOBCODE)-$(EXIF_YEAR)$(EXIF_MONTH)$(EXIF_DAY)-$(SEQUENCE).$(FILE_EXTENSION,,),
with the same uppercase error/bug: The Lisbon-20260131-0001.RAF instead of Lisbon-20260131-0001.raf.
Therefore, it seems to me that the bug is related with the "repetition" in the file number.

Expected behavior

Lower case in second-imported file of the same image (JPG+RAF). E.g. Lisbon-20260131-1001.RAF should be Lisbon-20260131-1001.raf.

Logfile | Screenshot | Screencast

No response

Commit

No response

Where did you obtain darktable from?

distro packaging

darktable version

5.5.0git139.4f4d6644-1+13622.1 and this is still true in 5.7.0git188.a8908e26-1+14497.1

What OS are you using?

Linux

What is the version of your OS?

Ubuntu 25.04

Describe your system

No response

Are you using OpenCL GPU in darktable?

None

If yes, what is the GPU card and driver?

No response

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

No response

Start in darktable's Copy & Import flow by reproducing the issue with the supplied FILE_EXTENSION pattern and JPG/RAF pair. Trace the filename handling when the second file is imported, including the repeated-file-number case; done means both imported extensions are lowercase, such as .jpg and .raf.