[Feature] Open only supported file types (web app)

Currently, we can open any file from "File"->"Open". We should allow only ".pd" files to be opened, as the web app supports them only. If we open other types of files either it doesn't open or gives weird errors.

Current:

We can upload any file (gives error):

Expected:

File chooser shows only PD files:

Alert message for unsupported file type: