FileTypeDetector not called for the .txt file, should I use FileTypeOverrider instead?

I'm creating a custom language support plugin. Source files have .txt extension, so an only way to differ them from simple txt is detecting whether they have necessary type using their content.

I register my file type in the following way:

<fileType name="DSF Source" implementationClass="com.onpositive.dsfedit.language.DSFFileType"
          fieldName="INSTANCE" language="DSF" extensions="txt" />

File type detector is registered as following:

fileTypeDetector implementation="com.onpositive.dsfedit.language.DSFFileTypeDetector"  order="first"/

Code:

public class DSFFileTypeDetector implements FileTypeRegistry.FileTypeDetector {
    @Override
    public @Nullable FileType detect(@NotNull VirtualFile file, @NotNull ByteSequence firstBytes, @Nullable CharSequence firstCharsIfText) {
        if (firstCharsIfText.length() > 10 && firstCharsIfText.charAt(0) == 'A' || firstCharsIfText.charAt(0) == 'I') {
            return DSFFileType.INSTANCE;
        }
        return null;
    }


    @Override
    public int getVersion() {
        return 0;
    }
}

Breakpoint inside `detect` isn't hit, even it I remove `extensions="txt"` from the first declaration

After some debugging I've found `fleTypeOverrider` extension, which is fired OK and defines my file type by content well. but it's result doesn't seem to be cached, so I'm caching them myself using file path as a key.

Am I doing smth wrong with fileTypeDetector, or fileTypeOverrider should be used in such a case?