With commit "EFI development work, merging code from Ubuntu" [1],
partman-efi does not format an EFI system partition (ESP) if any
filesystem is detected, in order to preserve existing ESP contents. Not
only FAT but also any other filesystem type and even Linux swap. However
the generated filesystem field in fstab is always set to "vfat"
regardless of the detected filesystem.

[1]
<https://salsa.debian.org/installer-team/partman-efi/-/commit/8cc1fddaead36f9d419f56cb46dd2c2d359e63f1>

This has a couple of issues:
- If the filesystem is not FAT, the generated fstab will fail to mount
the ESP.
- UEFI firmware may not support non-FAT filesystems.

What should partman do if the detected filesystem is not FAT ?
- Set the filesystem field in fstab accordingly ?
- Format the ESP (as FAT) ?
- Warn and prompt the user for what to do ?
- Other ?

Adding two related unanswered bug reports to the Cc: list.
This may happen with guided partitioning using an entire disk or manual
partitioning after deleting the existing partition or creating a new
partition table. I suspect it happens because partman-partitioning does
not properly clean partman device directory during these operations.

Other use cases are in manual partitioning or guided partitioning using
free space when re-using an existing partition, either non-EFI or EFI
with a filesystem other than FAT, as EFI.

To summarize the troublesome use cases:

* manual partitioning
- re-use an existing EFI partition containing a non-FAT filesystem
- re-use an existing non-EFI partition as EFI
- delete an existing partition and create an EFI partition at the same
location
- create a new partition table and an EFI partition at the same location
as an old partition

* guided partitioning using free space
- re-use an existing EFI partition containing a non-FAT filesystem

* guided partitioning using an entire disk
- create a new partition table and an EFI partition at the same location
as an old partition

Cleaning partman device directory when creating a new partition table or
deleting a partition would not fix the partition re-use use cases.

Should we enforce that an EFI partition must have a FAT filesystem ?
It has been reported that some UEFI firmware may support other
filesystems such as exFAT or NTFS.