Filename formatting silently fails under certain circumstances

[the-blank-x]

Step to reproduce: gallery-dl --no-download -f '\fF {asd}' https://skeb.jp/@wanwan_soba/works/48

Expected behavior: For it to say something like [skeb][error] FilenameFormatError: Applying filename format string failed (NameError: name 'asd' is not defined)

Actual behavior:

> gallery-dl --no-download -f '\fF {asd}' https://skeb.jp/@wanwan_soba/works/48
?
?

Additional information:
This only happens when the image URL doesn't have an extension:

> gallery-dl --no-download -f '\fF {asd}' https://skeb.jp/@wanwan_soba/works/48 https://http.cat/420.jpg
[1/2] https://skeb.jp/@wanwan_soba/works/48
?
?
[2/2] https://http.cat/420.jpg
[directlink][error] FilenameFormatError: Applying filename format string failed (NameError: name 'asd' is not defined)

Additionally, it also only happens if --no-download is passed:

> gallery-dl -f '\fF {asd}' https://skeb.jp/@wanwan_soba/works/48 https://http.cat/420.jpg
[1/2] https://skeb.jp/@wanwan_soba/works/48

[skeb][error] FilenameFormatError: Applying filename format string failed (NameError: name 'asd' is not defined)
[2/2] https://http.cat/420.jpg
[directlink][error] FilenameFormatError: Applying filename format string failed (NameError: name 'asd' is not defined)

[mikf]

This only happens when the image URL doesn't have an extension:

Do you think it would be OK to guess the filename extension for skeb files as webp?
It would speed up skipping previously downloaded files, and adjust-extension would correct webp to avif with -o downloader.headers.Accept=image/avif.

[the-blank-x]

Would it be OK to guess the filename extension for skeb files as `webp`? It would speed up skipping previously downloaded files, and `adjust-extension` would correct `webp` to `avif` with `-o downloader.headers.Accept=image/avif`.

Probably, though this feels like an unrelated issue since I'm 99% sure there are other extractors that return extension-less URLs (but I'm too lazy to find them)