Sitefinity by default is extensionless and given that some pages have .aspx and some don't, have you checked its not due to some faulty url-rewrite rules that may have been added to the web.config ?
Just to be on the safe side, you could check your PagesConfig.config file (located in /App_Data/Sitefinity/Configuration) that it looks like the following:
<?xml version="1.0" encoding="utf-8"?>
<pagesConfig
xmlns:config="urn:telerik:sitefinity:configuration"
xmlns:type="urn:telerik:sitefinity:configuration:type"
config:version="5.3.3900.0"
defaultFrontendTemplateId="00000000-0000-0000-0000-000000000000"
defaultBackendTemplateId="0000000-0000-0000-0000-000000000000"
enableBrowseAndEdit="False"
/>
(Naturally the 0000000-0000-0000-0000-000000000000 will be actual valid guids on your end)
Further more, you can swap your web.config out for one that comes out of the box to verify.
If some pages due to errors have been given extensions, you can simply remove them by using a snippet like this one from @selaromdotnet.
Jochem