Do not load plugins with file name extensions other than "dll"

Sometimes, in an attempt to temporarily disable plugin loading, users would append something to the extension (for example, “file.dll_”) and were surprised that the plugin still loaded. Now, plugins with such names will be ignored during the search.

The legacy of DOS times and 8.3 filename format creates a side effect: when searching for files with the mask “*.dll”, not only files like “file.dll” are returned, but also “file.dllabcd” (backward compatibility with the transformation “VeryLongFileName.VeryLongExtension” → “VERYLO~1.VER”). In the case of a .Net application, this is more likely to be a trap for the user rather than a pressing necessity to work in an old environment.