Save dialog for a decomposed layer stack suggested .tif instead of .xcf
When opening a tiff from disk, and then immediately selecting to "save" or "save as", the "save" dialog suggests the base tiff file name, with the extension ".xcf". By now, I'm guessing everyone has gotten use to this behavior (even if some people still think it's unfriendly behavior :) ).
However, after decomposing the newly-opened (and not yet saved as .xcf) tiff to RGB or LCH (my sample tiff only had one layer), and then selecting to "save" or "save as" the decomposed layer stack, the "save" and "save as" dialogs both offered to save the layer stack as a tiff, which surprised me. Yes, tiffs support multiple layers, but that's not the point.
When electing to use the GIMP-suggested "tif" file extension for saving the decomposed layer stack, a dialog popped up saying (as expected given that in GIMP "save" is only for XCF files) "The given filename cannot be used for saving", followed by "You can use this dialog to save to the GIMP XCF format. Use File→Export to export to other file formats."
This seems like a bug. It would be preferable if the dialog for saving the decomposed layer stack would just automatically use ".xcf" as the suggested file extension, instead of unhelpfully suggesting a non-supported file type for saving the layer stack.
This is for 2.10 and also for 2.99. The behavior also affects newly imported jpegs that haven't been saved as xcf files, and so likely affects all other "imported but not already saved as xcf" files.