I've been having trouble with MS Edge as a browser running Strawman.com. The blue bar with the "Home" button on it floats around all over the screen and yesterday it sat over a forum reply I was typing and caused the reply to be lost after I'd spent about 45 minutes typing it - I re-typed it but kept saving it into "Word" as I went in case it happened again. The next issue is that I can't scroll through more than about 5 items on the newsfeed without it stopping. I have to hit "refresh" to get that working again - constantly. The next issue is that this morning it said the site was down for maintenance, and yet I was able to log on using another browser (Mozilla Firefox) and it worked fine. Eventually I restarted "Edge" and it then loaded Strawman.com correctly. All this is to say that the problem you are experiencing with file attachments could be to do with the browser you are using. Try doing the same thing using a different browser. If the file attachments still don't work in a different browser (Chrome, Explorer, Firefox), then the issue is probably with the site. I've contacted Strawman about a number of issues (like the ones I've just mentioned in this reply) that turned out to be related to MS Edge (my browser) rather than the site. Of course, you could be using an android or apple phone, in which case I suggest trying to do the same thing with a laptop or desktop computer to see if it's a site issue or not. One of the issues with maintaining a site like this is that people will access it via so many different devices using so many different operating systems and these operating systems keep updating themselves, which often changes the way certain sites work. Plenty of headaches for Strawman and the developers I would imagine...
By the way, I don't attach files to any of my posts or straws. I just insert links to those files instead. That's because I had similar problems to the ones you described back in 2018 and I gave up on file attachments and just got used to using links instead. There was a period where you could "paste" images (like photos) into straws, and I managed to do that for a while, but then that didn't work either. The links still do. However, I have also noticed that not all links work for everybody. For instance, if you post a link from Commsec.com.au (CBA's share trading platform) or NABTrade or any other share trading platform, such as a link to a company announcement, the link won't work for other people unless they are logged into that particular trading platform. I have found that posting links from the ASX website works fine, because there is no paywall or login requirement, but any site that has a login requirement can be problematic for other people when they try to use your link.