Add ".oxt" file to the allowed attachment file format list of the Ask site

Currently .oxt file format is not allowed when we make comment and upload attachments. Oxt is the LibreOffice Extension format and should be allowed. For instance, see Error in "Chemical Engineering Symbols Gallery" extension - #12 by suokunlong.

I believe it was intentionally left out (and also from AskBot) in order to avoid diluting extensions uploads between this site and the extension site.

Well, we have plenty of extension on the extension website and many of them are not updated and out if date. Sometimes people may help to improve that extension or discuss that extension by doing some modifications and ask others to test. Thus it is a bad idea to not accept it in AskLibreoffice site.

I’m sure the premise is true but this not an argument to allow follow-up uploads all over the place on TDF-managed infrastructure. The project decided on a separate extension sites to upload/manage/comment/vote extensions — AskLibO (then AskBot) was at some point considered for the task but that was not chosen — so extensions.libreoffice.org is the place to use.

Moreover all content on this site is licensed under CC by-SA (see footer) so I’m pretty sure that re-uploading (improved) extensions here here is forbidden in many cases.