These are probably resources from some website that is served by their server without a Content-Type header entry known by your browser, i.e. the website did not identify (or misidentified) what type of data they are and how to handle them.
Your browser seems to download these kind of resources by default and because they don't have a proper file extension, they are treated as general files which is given the disk icon under some Windows configurations.
These files are normally useless as they're only meaningful when presented as part of the original webpage page. To be sure, you can open them up with a hex editor or notepad even and look for magic numbers (the first few characters of the file) or other strings that can identify the type of the files. For example, a GIF image begins with "GIF".
Note that this auto-download behavior is dangerous because malicious websites may be able to deposit executable files in your download folder this way.