I'm currently working in a web app for technology transfer offices and I've been avoiding at all costs to display a Modal window over another one that's already open.
To give more context, there is a file management component that basically allows you to upload or attach existent files in several sections of the web app, this component always appears as a modal window.
The dilemma I have right now is that I have another component that allows you to post "activity entries" (is a simple WYSIWYG text editor) which also appears as a modal and it has an option to attach files, so when you click the "attach file" button it should call the file management component in order to choose or upload the file you want to attach to that entry.
It is a capital sin to show a Modal Window over another one that is already open?
It would be better to extend the first Modal window and show both components at the same time (at least during file selection)?
Is there any guidelines from existing design language/system? Thanks!