always register text mime with mimetype

src/vs/editor/common/services/languagesRegistry.ts

@@ -114,7 +114,7 @@ export class LanguagesRegistry {
 		}
 
 		if (!primaryMime) {
-			primaryMime = 'text/x-' + lang.id;
+			primaryMime = `text/x-${lang.id}`;
 			this.mime2LanguageId[primaryMime] = lang.id;
 		}
 

src/vs/editor/common/services/modeServiceImpl.ts

@@ -545,7 +545,10 @@ export class MainThreadModeServiceImpl extends ModeServiceImpl {
 		// Register based on settings
 		if (configuration.files && configuration.files.associations) {
 			Object.keys(configuration.files.associations).forEach(pattern => {
-				mime.registerTextMime({ mime: this.getMimeForMode(configuration.files.associations[pattern]), filepattern: pattern, userConfigured: true });
+				const langId = configuration.files.associations[pattern];
+				const mimetype = this.getMimeForMode(langId) || `text/x-${langId}`;
+				
+				mime.registerTextMime({ mime: mimetype, filepattern: pattern, userConfigured: true });
 			});
 		}
 	}