Accept `.blob` files when importing Actual export (#785)

I'm not sure if this is something you want but it was a simple change so
I figured I might as well contribute it. This PR allows the user to
upload `.blob` files that they may have gotten from server's
`user-files/` folder. This can be useful if the user didn't export the
file but has server backups.

---------

Co-authored-by: Jed Fox <git@jedfox.com>