Once you have to write into the labels.txt file, why don't you put at the description the filename?

not really a satisfying solution, but maybe you are joking???

in fact I don't want to use the labels.txt at all!
I just want the filenames to be displayed in the popup, just as the SIG free.

Anyway, it seems that there is something wrong with your code since even with the "Read contents of caption files" enabled and the filename present in the labels.txt, this filename is not displayed.

So another idea?

There is nothing wrong with the code. It's how the SIG Pro works. It's not supposed to show the filename as the SIG free did.
There are many people that have their images named with strange names and numbering and it wasn't nice to be shown at the pop up. Apparently you are not one of them.
Furthermore, the labels.txt file allows you to do many things. The fact that the filename is shown at the beginning is because when you have to edit the file to know which title and description goes with which image and not because it has to be shown to the popup.
To achieve showing the filename to the popup needs further modification of the plugin so up to this point I'm afraid I don't have another idea besides the one I already offered.

There is nothing wrong with the code. It's how the SIG Pro works. It's not supposed to show the filename as the SIG free did.

At last, it is the response to the very thing I have asked in my first post...

Anyway, this should definitively be a parameter of the plugin : display the filename, yes or no.

We will consider adding this feature in a future version of the SIG Pro.

thanks