This is not specifically about graphene, but just about every Android phone I have had, even with AOSP, Lineage and other roms. Just about every single one I needed to rename my file to *.pdf to transfer to another phone, and then rename the extension again to origin. Its almost 2024, why is this still a thing? lol
Nobody else is bothered by this? omg
Bluetooth suggestion/request
Bismark I think I found the code that handles this stuff and confirmed what I guessed was probably going on.
If, for example, a phone running GrapheneOS wants to send a file to another phone running GrapheneOS, the share menu will show a send via Bluetooth option if the file matches any number of MIME types, including pictures, videos, documents, etc. This is why you had to rename files to send them.
When the receiving phone receives the file, it does a bunch of checks, but the important step is the Bluetooth app will try to find apps that claim they can open the file with the matching MIME type. If an app is found, the app is sent the file's location and the app opens the file. If no suitable app is found, the Bluetooth app just gives up, and it looks like it's supposed to display a message saying no suitable apps were found.
The Bluetooth "app" that does all of this is part of AOSP, so most Android phones would do it this way.
So, tl;dr, to fix this, all you need to do is find apps that claim they can open the appropriate MIME types that you plan on sending yourself.
The problem is not that the receiver cannot open them, but that the sender cannot send arbitrary files with any, even unknown, Mimetype over bluetooth.
On Android the best example is app backups. Launchers, players, keyboards, otp apps all backup stuff in some zip archive but named after the app, and are registered to open that "file type.
If android is so permissive here, why not allow sending any file over bluetooth?