Dev-Dep guide: mention AST third party requirements

Avoid the likes of:

```
Error:
ActiveStorage::RepresentationTest#test_representing_an_MP4_video:
Errno::ENOENT: No such file or directory - ffmpeg
    test/models/representation_test.rb:27:in `block in <class:RepresentationTest>'
```

Also note additiona macOS requirements in ActiveStorage guide.

[ci skip]

2 files changed, 33 insertions, 5 deletions

diff --git a/guides/source/active_storage_overview.md b/guides/source/active_storage_overview.md
index cb9acdadcf..3b7c23f2af 100644
--- a/guides/source/active_storage_overview.md
+++ b/guides/source/active_storage_overview.md
@@ -446,11 +446,11 @@ the box, Active Storage supports previewing videos and PDF documents.
 </ul>
 ```
 
-WARNING: Extracting previews requires third-party applications, `ffmpeg` for
-video and `mutool` for PDFs. These libraries are not provided by Rails. You must
-install them yourself to use the built-in previewers. Before you install and use
-third-party software, make sure you understand the licensing implications of
-doing so.
+WARNING: Extracting previews requires third-party applications, `FFmpeg` for
+video and `muPDF` for PDFs, and on macOS also `XQuartz` and `Poppler`.
+These libraries are not provided by Rails. You must install them yourself to
+use the built-in previewers. Before you install and use third-party software,
+make sure you understand the licensing implications of doing so.
 
 Direct Uploads
 --------------
diff --git a/guides/source/development_dependencies_install.md b/guides/source/development_dependencies_install.md
index 50274d700b..fdd560ccf3 100644
--- a/guides/source/development_dependencies_install.md
+++ b/guides/source/development_dependencies_install.md
@@ -376,3 +376,31 @@ command inside of the `activestorage` directory to install the dependencies:
 ```bash
 yarn install
 ```
+
+Extracting previews, tested in ActiveStorage's test suite requires third-party
+applications, `FFmpeg` for video and `muPDF` for PDFs, and on macOS also
+`XQuartz` and `Poppler`.. Without these applications installed, ActiveStorage
+tests will raise errors.
+
+On macOS you can run:
+
+```bash
+brew install ffmpeg
+brew cask install xquartz
+brew install mupdf-tools
+brew install poppler
+```
+
+On Ubuntu, you can run:
+
+```bash
+sudo apt-get update && install ffmpeg
+sudo apt-get update && install mupdf mupdf-tools
+```
+
+On Fedora or CentOS, just run:
+
+```bash
+sudo yum install ffmpeg
+sudo yum install mupdf
+```