So what the heck kind of file starts with the characters “CWS>+” ?

That would be a Flash SWF file!

If you’ve ever opened a binary file in a text editor and seen CWS at the start, you’re looking at a compressed SWF (Shockwave Flash) file. The CWS header means the file is zlib-compressed. Uncompressed SWF files start with FWS instead.

This is useful to know when you’re debugging file uploads, checking MIME types, or trying to identify unknown files on a server. The first three bytes of a file (the “magic number”) are a reliable way to determine the file type regardless of the extension.

Flash is long gone now, but you might still run into these files in legacy systems or old archives.