I am using chrome. But I tried IE and firefox, too. The browser seemed to have nothing to do with it.
I guess when a webpage, such as a forum post, is loaded, the "Multimedia plugins" filter will try to connect to youtube. If Youtube is available, it's ok. If not (for example, blocked), the webpage will be loaded quickly and fully, but with the webpage busy running as the following picture shows:
As there is no trouble editing the post or saving it, the busy running may not be noticed. When it comes to the file uploading, it becomes a problem.
It seems you won't experience the problem, with youtube-related plugins disabled or not, if visiting youtbe is ok (either not blocked or through ways to bypass the block) .
It happened in the background and IE and Chrome just told you it was "connecting". I didn't know what it was trying to connect to until I tried firefox. There wasn't "a non-succesful http code" or things like that. The browser just tried and tried ( usually more than twelve seconds, while on rare occasions just two seconds) before it gave up and became normal. Maybe a sniffing tool can give more information.
I guess it's the multimedia plugins filter that is responsible for this problem, not the multimedia plugins.
By the way, with Moodle 3.1, you can just paste into a forum post with the original format kept, such as the fontcolor:
Where is the color (the original color here is red)
It's a pity some format gets lost with Moodle 3.1.2 up. Can we have it back?