"The given filename does not have any known file extension. Please enter a known file extension or select a file format from the file format list."
indicate that the file-heif plug-in of GIMP hasn't registered the file type and common file name extension, so GIMP doesn't know about them.
The file lists for the GIMP packages from https://packages.ubuntu.com/mantic/gimp shows that the plug-in is supposed to be packaged, so it seems to have been built.
Could be that something is preventing it from running when GIMP starts up - this is when plug-ins are queried about e.g. what file type they are supposed to handle.
When you run GIMP from a terminal window, are there any interesting error messages?
The messages
"Opening '[...].HEIC' failed: Unknown file type"
and
"The given filename does not have any known file extension. Please enter a known file extension or select a file format from the file format list."
indicate that the file-heif plug-in of GIMP hasn't registered the file type and common file name extension, so GIMP doesn't know about them.
The file lists for the GIMP packages from https:/ /packages. ubuntu. com/mantic/ gimp shows that the plug-in is supposed to be packaged, so it seems to have been built.
Could be that something is preventing it from running when GIMP starts up - this is when plug-ins are queried about e.g. what file type they are supposed to handle.
When you run GIMP from a terminal window, are there any interesting error messages?