Renaming part of title attachment

You have two options : 

1 - Use html in a formula :

"set the file to rename here";
var file := first(files(this));

"set the new file name here";
var newFileName := "test.jpg";

var uniqId := "btn" + now();
html("
<div id='" + uniqId + "' classe='component' title='' onclick='renameFile()'
    style='width:100%;padding:0px;text-align: center; -moz-user-select: none;-webkit-user-select: none;-ms-user-select: none;user-select: none;'>
    <div class='nx-button-text blue'>Run code</div>
</div>

<script>
    //  code to setup the button
    var me = document.getElementById('" + uniqId + "');
    me.closest('.html').style = 'padding:0px;height:34px';
    me.closest('.stringeditor').style = 'border-width: 0px;'
    var label = me.closest('.editor-html').querySelector('label')
    me.querySelector('.nx-button-text').innerText = label.innerText;
    label.style.display = 'none';

    // function to rename file
    function renameFile() {
        var f = util.parseNIDFile('" + file + "');
        database.renameFile(f[0] + f[1], f[2], '" + newFileName + "', (err) => { if (err) alert(err); });
    }
</script>")

This formula creates a button similar to those of Ninox and calls the renameFile function when you click on it.

2 - Uses NativeJS : 

Put this code on button formula.

"set the file to rename here";
var file := first(files(this));

"set the new file name here";
var newFileName := "test2.jpg";

#{
    var f = util.parseNIDFile(file);
    database.renameFile(f[0]+f[1], f[2], newFileName, (err) => {if (err) alert(err);});
}#

thomas said:
Unfortunately it gives me an error. I guess it could be because the content of the "title" field is generated by a formula

 That is your problem. You can not modify a formula field by forcing a value into it.

You have to two options:

1) Since “title” is a formula field, You can replace the number in the number or text field that makes up the part of the name that you need to change.

or 

2) You can write the results to a new field.

Looking at the starting post, you have the field 'Interne Nr.' Is that a user editable field? If not then you can try the following in your Titel formula:

let x := if 'Nr zuweisen' = null then
    'Interne Nr.'
else
    'Nr zuweisen'
end;

field1 + "-" + "Nr"+ x + etc...

Here we create variable that checks if 'Nr zuwisen' is empty and if it is then it will use the value in 'Interne Nr.' else it will use the value in 'Nr zuwisen'.

Then we use that variable in the formula that creates the Titel.

Sorry this is taking so long. Thanks for putting up with my silly questions that you have already answered in your original post.

I think I know a solution. You have two formula fields (Titel, Interne Nr.). Which means you need a new text field if you want to change Titel with what is in Nr zuweisen.

In the button you can try something like:

'new text field := if 'Nr zuweisen' = null then
    Titel
else
    replace(Titel,'Interne Nr.','Nr zuweisen')
end

Or you can put the formula into a new formula field (without the ‘new text field’ part) and the end user doesn’t have to press a button.