In some rare cases, we can't add templates.

In some rare cases, we can't add templates.
The client couldn't upload templates on Jira 7.3.x and 7.6.3.
Check this support for more info.

The conclusions I got is that Jira needs a DIV with a specific id retrieving the Attachment Max Size and it also needs the Template Input to have a specific class.

<form ...>
<div id="attach-max-size" class="hidden">$attachmentMaxSize</div>
...
<input class="ignore-inline-attach" ...>
</form>