Maybe this explains what you see:

Attachments and notes are not really linked (known feature request 0021733), they are just shown inline, see 0021727.

If a user submits a note with an attachments within a specified number of seconds (default is 3) the attachment seems to be linked to the note.
This can be configured by setting $g_issue_activity_note_attachments_seconds_threshold = 3;

Hi atrol,

thanks for the quick reply! Setting $g_issue_activity_note_attachments_seconds_threshold seems to handle the issue.
What might happen if I increase the variable to much, maybe to 10 or 15?

Just to be sure I do not cause any further problems by manipulating its value?

Best regards
Steve

One more comment on this:
As I said it seems to influence the issue but does not solve.
I set $g_issue_activity_note_attachments_seconds_threshold = 100; but one of three attachments came up in a new post, two where in line with my post so the issue still exists. Upload did not exeed 100 seconds so there seems to be another reason for the behaviour.

What might happen if I increase the variable to much, maybe to 10 or 15?

Should not hurt, could just be a bit confusing if someone wants to add a separate attachment just a few seconds after he attached the previous attachment.
Hardly a real use case

I set $g_issue_activity_note_attachments_seconds_threshold = 100;

100 seconds is too much IMO.

but one of three attachments came up in a new post

Maybe this attachment was added by another user.

Ok, thanks I understood its philosophy.

I think realizing the link between attachment and note would be best solution.
What do you think, can we implement this? Feature request is quite old. Shouldn't be a too big thing, is it?

Another option to fix dirty was to set the date of upload ("date_added") equal to the date of post. Could have this code in procedure of klicking the button "add note" or something.

I think realizing the link between attachment and note would be best solution.

Right

What do you think, can we implement this?

Submitting a patch is always a good idea, as it increases the chances of improvement eventually making it into MantisBT core. All contributions are welcome and greatly appreciated.

You can send us a Pull Request on our Github repository [1]
Please make sure that your submissions adhere to our Coding Guidelines [2], if they don't your patch might be rejected.

It will not become part of a minor release, as database schema changes are needed, so it will take some time until your development will become part of the official distribution.

[1] https://github.com/mantisbt/mantisbt
[2] http://www.mantisbt.org/wiki/doku.php/mantisbt:coding_guidelines