Can upload file from Teams chat files to Posts but not to Shared

I was working on an Excel file shared in a chat with members of a small workgroup. When it was ready to share with the larger team, I navigated to the appropriate Team > Channel, created a Post, clicked on + > Attach file > selected the file (which had been modified recently, so displayed on the pop-up).

The team lead asked me to save it in a particular folder in the team's files, so I navigated to Shared (formerly Files)... and did not see it. I could briefly see it if I selected the "In messages" filter, but then after a moment that bugged out and displayed the "Something went wrong" error screen. This behavior persisted (briefly visible file list, then error screen) even after quitting Teams via the task bar and opening it back up.

I did not want to create a separate version (especially now that the original file was linked in the Post), so I tried a few different methods to get it into Shared.

• I tried copying a link, but couldn't figure out a good way to paste it into Shared.
• I tried pasting it in a new Link, but that preserved it as a link/URL rather than a file -- serviceable but awkward.
• I checked whether there was an easier way to do it by opening in the web version, where Files is still active rather than Shared (same behavior, except no "In messages" filter so I couldn't test that).
• I poked around in SharePoint but didn't see anything interesting.
• I tried uploading a copy from within Shared by pasting the filepath from the "Save a Copy" dialog in Excel...

...and that's when I noticed a possible cause of the issue: another member of the original workgroup chat had created the file, and it was living in her Teams chat files, not mine. I then had her try doing it by navigating to her Teams chat files and selecting her copy, and then hallelujah, it appeared in Shared. I was able to drag it to the appropriate folder normally.

Ultimately, I see two questions:

1. Why does the file appear when the "In messages" filter is selected, but not otherwise?
2. Why can I add a file saved in someone else's Teams chat files to a Post, but I can't upload it in Shared?

Bonus: any idea why the "In messages" filter caused an error message to display?