[Bug]: Undetermined meaning of "mimetype" in Flows

⚠️ This issue respects the following points: ⚠️

• This is a bug, not a question or a configuration/webserver/proxy issue.
• This issue is not already reported on Github OR Nextcloud Community Forum (I've searched it).
• Nextcloud Server is up to date. See Maintenance and Release Schedule for supported versions.
• I agree to follow Nextcloud's Code of Conduct.

Bug description

Meaning of mimetype in not determined and not documented.

• In some cases mimetype is defined by filename (extension)
• In other cases, it is defined by file content

Steps to reproduce

1. Using files_accesscontrol flow, block images by mime-type
2. Having a local image file, for example, image.png, rename it to image.notpng
3. Upload the image on the server (success)
4. Remove the image from the server (blocked)

As a result, it is possible to upload a file that is not possible to delete due to block from files_accesscontrol.

Expected behavior

If mimetype is determined by file content, uploading an image should not have been allowed even it has extension .notpng.

If mimetype is determined by filename (extension), it should be allowed to delete the uploaded file because it has a non-image extension.

If this is expected behavior, it should be documented.

Nextcloud Server version

master

Start with the files_accesscontrol flow and reproduce the renamed-image scenario described in the issue. Trace how mimetype is determined during upload and deletion, then establish whether the behavior is based on the filename or file content. Done means the behavior is consistent or its precise meaning is documented.