There's a quick way and a more secure way.
The easiest option would be to allow uploads where the MIME type doesn't match the extension. You can do this by adding the following in your LocalSettings.php (making sure no conflicting statement is on another line:
$wgStrictFileExtensions = "false";
Disabling $wgStrictFileExtensions is a serious security risk as stated in the Mediawiki documentation, so only do this if you trust all users with upload rights. It will give users a warning about the file type, but upload the file anyway. To not have MediaWiki check at all, outright disable $wgVerifyMimeType with:
$wgVerifyMimeType= "false";
A more proper way is to have MediaWiki know 'application/sla' is OK for .oft files. The default MIME-type database expects files with that extension to be ODF formula templates.
You can change that by supplying MediaWiki with your own definition file for MIME types.
In Linux, you might have - in Debian for example, it's /etc/mime.types. You can add the line
application/sla oft
And make sure LocalSettings.php has $wgMimeTypeFile set to link to that file. In the Debian example:
$wgMimeTypeFile = "/etc/mime.types";