Open old AI files

[defaulti541xg3osspg]

Hello dear AI community, I would like to open an old AI file from 2012 and I always get the message "File has an unknown format and cannot be opened" I have always worked with current AI files. How can it be that AI can no longer open its own files?
Thank you for the answer in advance.

[Monika Gause]

Illustrator is perfectly able to open old files.

How did you save that file?

Where did you save it?

How did you transfer it to the current computer?

Is the filesize reasonable?

Does the file have a .AI extension? (as a Mac user I know that sometimes you just omit/ignore/disregard the extension)

How do you open the file (just double click or do you use the Open dialog box)?

What's in the file?

Did the hard driver perhaps get corrupted at some time and did you have to recover it?

[defaulti541xg3osspg]

How did you save that file?

Quite normal under Illustrator
Where did you save it?

On my computer
How did you transfer it to the current computer?

Old and new computers connected, Mac did the rest
Is the filesize reasonable?

That seems to be independent of the size, all files do not open for about 2012
Does the file have a .AI extension? (as a Mac user I know that sometimes you just omit/ignore/disregard the extension)

No, the .ai is missing from these files. Tried to put it behind it - but it still doesn't open.
How do you open the file (just double click or do you use the Open dialog box)?

Tried both options
What's in the file?

In this case a graphic representation. But apparently it is irrelevant, as all files cannot be opened before 2012.
Did the hard driver perhaps get corrupted at some time and did you have to recover it?

No, not that I know of

[Ton Frederiks]

Open the file in a text editor, check if it starts with %PDF-1.5 or similar.