maybe a google search? nope, no right answers on the first page. or the second page. in fact, the results are flooded with websites claiming to decode file extensions.
i think my favorite description so far is "AXD files are typically a type of binary file used by certain software, and many software packages can share a common file extension. In the case of files..."
@tubetime and it probably doesn't help that it shares extension with some sort of visual studio file. I went to my old stand by, but can't help: https://fileinfo.com/extension/axd