Only UTF-8 (sql) files are currently supported.

Describe the solution you'd like
A drop-down in the file-open dialog, like in the Windows version:

Lazarus has simple-to-use components for creating a custom dialog which would then look the same on all platforms:
https://forum.lazarus.freepascal.org/index.php/topic,69880.0.html

Additional context
The save-as dialog does not necessarily need such a selector. The Windows version only offers a line-feed selector instead:

Related:

• https://www.heidisql.com/forum.php?t=44351
• df6abc9