Unable to export pptx with a specified output file name.

[namidapoo]

Hi, thank you for this project!

When I try to export a pptx file with a specified output file name, it fails.

# failed
$ slidev export --output output/example.pptx --format pptx

The output when executed will be an error as follows.

> slidev export --output output/example.pptx --format pptx

  ●■▲
  Slidev  v0.50.0-beta.7 

  theme       @slidev/theme-seriph
  css engine  unocss
  entry       /Users/User/Projects/slidev/talks/slides/slide1/slides.md
  ● rendering...[Error: EISDIR: illegal operation on a directory, open 'output/example.pptx'] {
  errno: -21,
  code: 'EISDIR',
  syscall: 'open',
  path: 'output/example.pptx'
}
 ELIFECYCLE  Command failed with exit code 1.

If I only specify the output file name or only export as a pptx, there are no problems.

# successed
$ slidev export --format pptx
$ slidev export --output output/example.pptx

Also, if I remove the .pptx extension, it will work, but an empty folder will also be created.

$ slidev export --output output/example --format pptx

How can I export with a specified output file name, in the same way as when the format is pdf?