Short answer
Any file type. Aktar uploads the original file unchanged, without compression or resizing, and sets the correct Content-Type so browsers show images, PDFs and videos inline.
The library shows previews for images, PDFs, text and Markdown files; other files get an icon.
Images copied to the clipboard without a file behind them, such as a screenshot taken with ⌃⇧⌘4, are saved as PNG before uploading.