Fix string formatting in FileConversionException error message

[yushihang]

Problem

The error message formatting in FileConversionException class is incorrect due to a missing f-string prefix, preventing proper display of the converter type name.

Changes

• Added f prefix to enable string interpolation in error message
• Ensures proper display of converter class name in exception messages

Testing

• Verify exception message correctly displays converter class name
• Ensure other exception messages are formatted correctly

Related Issues

None

Notes

This is a simple string formatting fix with no functional logic changes.