isFileOfType(): add javadoc, run detectors when asked about PlainTextFileType (IDEA-CR-48346)

GitOrigin-RevId: a70ba89a9b352ca4e968259feb4e57872344286d

platform/core-api/src/com/intellij/openapi/fileTypes/FileTypeRegistry.java

@@ -32,6 +32,13 @@ public abstract class FileTypeRegistry {
 
   public abstract boolean isFileIgnored(@NotNull VirtualFile file);
 
+  /**
+   * Checks if the given file has the given file type. This is faster than getting the file type
+   * and comparing it, because for file types that are identified by virtual file, it will only
+   * check if the given file type matches, and will not run other detectors. However, this can
+   * lead to inconsistent results if two file types report the same file as matching (which should
+   * generally be avoided).
+   */
   public abstract boolean isFileOfType(@NotNull VirtualFile file, @NotNull FileType type);
 
   @Nullable

platform/platform-impl/src/com/intellij/openapi/fileTypes/impl/FileTypeManagerImpl.java

@@ -996,9 +996,12 @@ public class FileTypeManagerImpl extends FileTypeManagerEx implements Persistent
 
   @Override
   public boolean isFileOfType(@NotNull VirtualFile file, @NotNull FileType type) {
-    if (type instanceof UnknownFileType) {
+    if (type.equals(PlainTextFileType.INSTANCE) || type.equals(UnknownFileType.INSTANCE)) {
       // a file has unknown file type if none of file type detectors matched it
-      return file.getFileType() instanceof UnknownFileType;
+      // for plain text file type, we run file type detection based on content
+
+      //noinspection SSBasedInspection
+      return file.getFileType().equals(type);
     }
 
     if (file instanceof LightVirtualFile) {