Use application/config/config.php :

https://manual.limesurvey.org/Optional_settings#Note

'config'=>array(
    'debug'=>0,
    'debugsql'=>0,
    'allowedtemplateuploads'=>'gif,ico,jpg,png,css,js,map,json,eot,svg,ttf,woff',
)

Hi Denis, I guess many users will not find this workaround. Is there an objection to add the named extensions to the file?

It's not a workaround : official manual.

I see no reason for not allowing these files types to be accepted at template upload. They can do no harm and I have seen several users complaining about not being able to upload e.g. font files.

Then : why not adding : pdf, html, txt, mp3 ...
Just deactivate the settings.

Font files are nowadays rather common parts of a template. PDF and the like are not.
BTW, a pull request is here: https://github.com/LimeSurvey/LimeSurvey/pull/310

Fix committed to master branch: http://bugs.limesurvey.org/plugin.php?page=Source/view&id=15092

Fix committed to 2.06 branch: http://bugs.limesurvey.org/plugin.php?page=Source/view&id=15093

Please close this bug report, since it was solved.