`_brand.yml`: `typography: fonts: source: file` option fails

Bug description

The source: file option does not work for me as expected when trying to specify a custom font in a brand.yml. It insists the source field's value must be google (but I do use it successfully with source: system, for example).

Steps to reproduce

Copy follwing files to same test directory

_brand.yml

typography:
  fonts:
    - family: Amplitude-Light
      source: file
      files: Amplitude-Light.ttf
  base: Amplitude-Light
  headings: Amplitude-Light

_quarto.yml

title: Test

my.qmd

# Foo bar

- Test
- Foo
- Bar

Amplitude-Light.ttf

(not included here, but could be replaced by other suitable font file)

Compile

quarto render my.qmd --to html

Actual behavior

I am getting the following error. Notice in particular the claim that Field "source" has value file, which must instead be 'google', even though I use source: system with system fonts quite successfully on the same machine.

ERROR: Brand validation failed for /home/me/font_test/_brand.yml.

In file _brand.yml
(line 5, columns 7--12) property name files is invalid
4:       source: file
5:       files: Amplitude-Light.ttf
        ~~~~~~
6:   base: Amplitude-Light
ℹ The error happened in location typography:fonts:0:files.


In file _brand.yml
(line 4, columns 15--19) Field "source" has value file, which must instead be 'google'
3:     - family: Amplitude-Light
4:       source: file
                ~~~~~
5:       files: Amplitude-Light.ttf


Stack trace:

In file _brand.yml
(line 5, columns 7--12) property name files is invalid
4:       source: file
5:       files: Amplitude-Light.ttf
        ~~~~~~
6:   base: Amplitude-Light
ℹ The error happened in location typography:fonts:0:files.


In file _brand.yml
(line 4, columns 15--19) Field "source" has value file, which must instead be 'google'
3:     - family: Amplitude-Light
4:       source: file
                ~~~~~
5:       files: Amplitude-Light.ttf

    at readAndValidateYamlFromFile (file:///opt/quarto/bin/quarto.js:20482:15)
    at eventLoopTick (ext:core/01_core.js:175:7)
    at async projectResolveBrand (file:///opt/quarto/bin/quarto.js:40049:27)
    at async resolveFormats (file:///opt/quarto/bin/quarto.js:80545:23)
    at async renderContexts (file:///opt/quarto/bin/quarto.js:80377:21)
    at async renderFileInternal (file:///opt/quarto/bin/quarto.js:85654:20)
    at async renderFiles (file:///opt/quarto/bin/quarto.js:85604:17)
    at async renderProject (file:///opt/quarto/bin/quarto.js:86020:25)
    at async Command.actionHandler (file:///opt/quarto/bin/quarto.js:90654:32)
    at async Command.execute (file:///opt/quarto/bin/quarto.js:8111:13)

Expected behavior

Successful compilation with local font file.

Your environment

• IDE: vim & terminal
• OS: Debian

Quarto check output

Quarto 1.6.42
[✓] Checking environment information...
      Quarto cache location: /home/flo/.cache/quarto
[✓] Checking versions of quarto binary dependencies...
      Pandoc version 3.4.0: OK
      Dart Sass version 1.70.0: OK
      Deno version 1.46.3: OK
      Typst version 0.11.0: OK
[✓] Checking versions of quarto dependencies......OK
[✓] Checking Quarto installation......OK
      Version: 1.6.42
      Path: /opt/quarto/bin

[✓] Checking tools....................OK
      TinyTeX: (not installed)
      Chromium: (not installed)

[✓] Checking LaTeX....................OK
      Using: Installation From Path
      Path: /usr/bin
      Version: 2024

[✓] Checking basic markdown render....OK

[✓] Checking Python 3 installation....OK
      Version: 3.12.7
      Path: /usr/bin/python3
      Jupyter: 5.7.2
      Kernels: python3

[✓] Checking Jupyter engine render....OK

[✓] Checking R installation...........OK
      Version: 4.4.1
      Path: /usr/lib/R
      LibPaths:
        - /usr/local/lib/R/site-library
        - /usr/lib/R/site-library
        - /usr/lib/R/library
      knitr: 1.48
      rmarkdown: 2.27

[✓] Checking Knitr engine render......OK