Allow importing files with MIME type "application/octet-stream"

Sometimes Android reports the MIME type as "application/octet-stream" even if the file has a .json extension (specifically, on Android 8 and 9 for backups exported outside the Download folder). Previously, such files could not be selected in the file picker when importing.

As a workaround, allow files with that MIME type.

@​devs: Should I also port this fix to the other apps that have a JSON import feature?

What is it?

• [x] Bugfix
• [ ] Feature
• [ ] Codebase improvement

Before/After Screenshots/Screen Record

No response

Fixes the following issue(s)

• Fixes #88.

Acknowledgement

• [x] I read the contribution guidelines.