How to delete an attachment? Why is PDF broken?

Well, that will be a lot:
starting with most of RegularLabs' plugins, we have SobiPro, OSG Seminar Manager, JCE Editor and Mediabox, Switch Editot, Litle Helper, JotCache, XT Adaptive Images, AutoArchive, Kunena Forum, JCH Optimize, JS Async&Defer, Phoca OpenGraph and OSMap acitvated Hope the delinquent is already on your list of intervening plugins.
Thanks for your help!

@hoffi:
I can reproduce the small "icon" in isis backend on Joomla! 3.8.3. (see image) php 7.1.11

in frontend too

When I take away (e.g. in protostar-Frontend) max-width:100% the icon is in his normal width (template.css line 50).
The same in isis-backend template.css line 50 max-width: 100%.
But I don't know which effects are on other Places!

Thanks Jojo, I think I found it. It's a bug in new Firefox: 100% of 16px = 4px - I hope these developers will never write financial software! :ohmy:
It seems the guys have completely changed the behavior. Since the first browser a percentage value on an image tag was related to the image size. Now it seems to be relative to enclosing tag. But the result of 4px keeps a big secret...

As workaround you can edit backend.css around line 231 by adding max-width: none; to get final block like
I'll think about a solution in next version to support such stupid browsers...

btw: Now I wouldn't be surprised if this browser also modifies pdf files... :S

I can't find a backend.css in any com_jem directory. What did you mean exactly?

css files are in media/com_jem or if you use the responsive jem in template override folders