I know it is possible to decompile SharePoint web parts if you have their .dll or .wsp files (I have successfully done this myself using tools like DotPeek or this method), but what if you are left with just a ".webpart" file? Renaming it to a .zip doesn't work, and it's not a supported file type in a decompiler either. Is there any way to resurrect the source from these?
Open it with Notepad or your favorite XML editor.
See Web Part Properties - part 3 - the .webpart file by Wictor Wilen for an example.