What version of WordPress?
Let me also add, that only the network admin pages are affected.
The normal admin pages and the normal web pages are not affected.
Does it work if you activate it on a single site, instead of network activating?
Yes, without “network activate” it works.
However trying your plugin out it seems that it uploads it not into the media upload folder
“http://domain.com/subblog/files/2013/06/font.ttf”
but instead into
“http://domain.com/subblog/wp-content/uploads/fonts/font.ttf”
which is wrong because such a subfolder does not exist.
I also don’t like that you don’t announce which folder the font has been uploaded to.
Additionally I don’t like that your plugin’s name is “Font Uploader”, yet your plugin not only uploads fonts, but also allows to apply the uploaded fonts (and those fonts ONLY) to elements.
That way, one needs to use different plugins (i.e. your plugin and someone else’s plugin) depending on whether one wants to set, say, the header with font hosted by Google or a with an uploaded font or with a standard font like “Arial” or a font supplied by the theme.
You should stick to the font uploading capability only or make two plugins.
Hmm, okay I will have to look into that.
Why’s it a problem that the location of the fonts isn’t announced? The only time I can think of that being an issue is if you want to upload the manually.
In regards to the name, a plugin that uploads fonts but doesn’t let you apply them is useless.