By default, Adobe Illustrator saves files in PDF format. If the file has not been saved in any strange way, you can use ImageMagick directly to create a thumbnail. For example:
convert file.ai -thumbnail 250x250 -unsharp 0x.5 thumbnail.png
Note. If a file has multiple artboards (which are interpreted as PDF pages), it will generate multiple files or, if they are saved as GIFs, animated GIFs.
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