Is there an easy way to add, modify, or remove what files or URLs are associated with Apps?
RCDefaultApp.


UPDATE

As of 2020, RCDefault App seems to have been abandoned.  There is another FOSS alternative - SwiftDefaultApps which is available on GitHub and via Homebrew.  It's described as a "Replacement for RCDefaultApps" and testing has shown that it functions pretty much the same way as the original.  It works on macOS 10.12.x (Sierra) through 10.15.x (Catalina).



There's a free (as in beer) application that you can use to change how macOS behaves with respect to application associations and URL, protocol, and media handling:  RCDefault App.


  RCDefaultApp is a Mac OS X 10.2 or higher preference pane that allows
  a user to set the default application used for various URL schemes,
  file extensions, file types, MIME types, and Uniform Type Identifiers


Set Default Apps

One of the most convenient uses of this application is to set your default app for common protocols like web browsing or email (setting your default browser or mail apps) or switching FTP from Finder to Firefox or Chrome.  



File (Extensions) by Application

One of the more useful (and global) features is to find out what file extensions are "assigned" to a particular application.  For example, here we can see what's associated with Acrobat Pro.



Applications by Extension

The flip side to seeing what's assigned to a particular application is to see what application is assigned to a particular extension.  In this case PDF is assigned to Acrobat Pro.  From here, we can change its association to another application (like Preview).



Disable an App

One of the more useful tools (extremely useful in Lab or shared environments) is to disable certain apps, extensions, or protocols from ever being opened.  For example, in the example below, Spotify can be disabled and will not open.



Installation &amp; Use

It's an older app and it hasn't been updated since 2009; however, I recently tested it on both El Capitan and High Sierra.
Tested still working on Mojave 10.14.1 at Oct 18.

After downloading and mounting the DMG, simply copy the file RCDefaultApp.prefPane to either of the following directories:


/Library/PreferencePanes/ - will be available systemwide for all users
~/Library/PreferencePanes/ - only availabe to your user profile


Open System Preferences and launch the utility



Pricing

It's FREE as in beer, but if you do find the utility useful, I encourage you to make a donation.  Please note that I am not in any way affiliated with the developers or the software referenced here in any way.