Hey, I'd have some ideas for workarounds:
1. Is there a way to put a "default directory" for that file explorer? ( Considering I'm not using the way to just open the file explorer, I'm using app.executeCommand(3075); and it opens the " save an animation preset " file explorer)
I just tested this successfully: so, the file explorer always opens up from the last folder. And that also goes for from where you applied a preset. So the idea would be to create a dummy ffx preset from where you want the default directory to be and apply the preset to your comp. Then delete it from the comp and from the directory. Now execute the command to save the preset and the file explorer should open from the directory you applied the preset from.
2. Is there a way to to customize that window? Like changing the extension, the title, etc. ) just like when we would do that : new File("Default File Name").saveDlg(["Title"],["PNG Files:*.png"]);
Maybe you could copy a text to your clipboard and then paste it once the dialog opens. But this would be really hacky and I'm not sure whether it would work. I haven't been able to make it work yet when using AE's paste command. Maybe it'll work when forcing keyboard commands (like Ctrl+V)? I once have been able to make an enter keypress work on one these dialogs by simulating an enter keypress with AutoIt on Windows and via AppleScript on Mac right before I executed the dialog. Maybe the same would apply for pasting? Not sure. Also, this would, of course, be only to give it a default name. Other than that, I don't think there is a way to do any other modifications.
3. Is there a way to know what the user wrote inside of that file explorer? ( like the name he put to the file )
Hm, not sure. The only thing I could think of right now would be to scan all possible folders first and then after the file has been saved to look for a new ffx file. But that might be quite insane since there would be probably too many folders on your computer to scan. Another interesting note though: once you save an animation preset, it will be listed with its name under "Recent Animation Presets". The menu command ID for the most recent preset is "2460". However, I don't know how to get the name from the command ID (usually it's just the other way around ;)). If you could somehow get the name, you would know what the user typed in.