Hide all files with a specific extension from Finder
If you just want to modify the saved search, copy it somewhere from /System/Library/CoreServices/Finder.app/Contents/Resources/CannedSearches/Today.cannedSearch, show package contents and open search.savedSearch in a text editor, and add this to the first RawQuery key:

&amp;amp;&amp;amp; kMDItemFSName != *.log &amp;amp;&amp;amp; kMDItemFSName != *.upload &amp;amp;&amp;amp; kMDItemFSName != *.json

You can drop it on the sidebar by holding command.