Though, while I was trying to attach the databases I was getting an error message of the following type:
Msg 3415, Level 16, State 3, Line 1
Database [database_name] cannot be upgraded because it is read-only or has read-only files. Make the database or files writeable, and rerun recovery.
Msg 1813, Level 16, State 2, Line 1
Could not open new database [database_name]. CREATE DATABASE is aborted.
As the error message was saying, I checked the permissions of the database files and ensured that they were not read-only. Also, the service user account running the SQL Server instance had full access on the files.
As I did not have much time for fully troubleshooting the issue, I provided full access to "Everyone" on all the database files I wanted to attach, and tried again.
Guess what? It worked :)
After the databases were successfully attached, I removed the full access from the "Everyone" entity and so everything was back to normal!