For some strange reason on my machine Signal had taken over as the default program for HTML files. Or, in other words, if I clicked on an HTML file such as index.html then Signal would automatically open instead of the file being opened in Firefox or Gedit or something reasonable. How I managed to get a messaging program to do this is a mystery to me, fixing it is quite easy though.
- Go to the file you want to open in Nautilus (The Gnome file manager) and find the file that is giving you problems.
- Right click on the file and choose Properties.
- Open the third tab of properties called Open With.
- Find the program you want to open the file with an select it.
- Click Set As Default on the bottom right to set that program as default.