fix(Page.screenshot): prioritize screenshot type over filename extens…

…ion (#1526)

For the `page.screenshot` command, this patch starts prioritizing `options.type` option over the type inferred from `options.path`

Fixes #1492.

lib/Page.js

@@ -614,19 +614,20 @@ class Page extends EventEmitter {
    */
   async screenshot(options = {}) {
     let screenshotType = null;
-    if (options.path) {
+    // options.type takes precedence over inferring the type from options.path
+    // because it may be a 0-length file with no extension created beforehand (i.e. as a temp file).
+    if (options.type) {
+      console.assert(options.type === 'png' || options.type === 'jpeg', 'Unknown options.type value: ' + options.type);
+      screenshotType = options.type;
+    } else if (options.path) {
       const mimeType = mime.lookup(options.path);
       if (mimeType === 'image/png')
         screenshotType = 'png';
       else if (mimeType === 'image/jpeg')
         screenshotType = 'jpeg';
       console.assert(screenshotType, 'Unsupported screenshot mime type: ' + mimeType);
     }
-    if (options.type) {
-      console.assert(!screenshotType || options.type === screenshotType, `Passed screenshot type '${options.type}' does not match the type inferred from the file path: '${screenshotType}'`);
-      console.assert(options.type === 'png' || options.type === 'jpeg', 'Unknown options.type value: ' + options.type);
-      screenshotType = options.type;
-    }
+
     if (!screenshotType)
       screenshotType = 'png';