As part of the refactoring which took place in 2.11.0 (see MantisBT master 26c869f6 / 0023774) the helper_array_transpose() call was removed from the file_attach_files() function; this is probably why it's no longer working for you.

That being said, the correct way to attach files now would be to rely on the relevant Command (i.e. IssueFileAddCommand or possibly IssueNoteAddCommand); have a look at bugnote_add.php for example usage.

Thank you very much @dregad. I really wanted to fix this cleanly.
I used the command, but I had a problem submitting the note in private state with the file-upload but I fixed the problem by executing both commands:

• 1.: IssueFileAddCommand to upload the file
• 2.: IssueNoteAddCommand with only the note in private view state

Now it works as I intended:)

Please note that even if the note is private, attachments are public

I did not know that, thank you.

You might want to read 0025935 and 0009802.