fix(AttachmentsPage): handle application/octet-stream

[GeopJr]

fix: #1311

As discovered on the linked issue, Pleroma seems to set supported_mime_types to application/octet-stream in an attempt to communicate that everything is supported. However on Tuba, we either use whatever the instance returns or use the built-in list (which is from Mastodon).

That means that since Akkoma returns nothing, Tuba internally accepts everything mastodon does. But for Pleroma's case, when the user selects an image/jpeg file, Tuba will see that image/jpeg is not in the supported mime types the instance returned which is just application/octet-stream and tuba rejected it silently.

This PR makes it so if the only supported mime is application/octet-stream then it should be ignored and instead use the internal list.