ChangeTheHeaders

During the most recent episode of The Talk Show, Jason Snell brought up a weird issue that I started running into last year. On my Mac, sometimes I’d drag an image out of a web page in Safari, and I’d get an image in WebP format. Sometimes I wouldn’t care. But usually when I download an image like that, it’s because I want to publish (or merely host my own copy of) that image on Daring Fireball. And I don’t publish WebP images — I prefer PNG and JPEG for compatibility.

What made it weird is when I’d view source on the original webpage, the original image was usually in PNG or JPEG format. If I opened the image in a new tab — just the image — I’d get it in PNG or JPEG format. But when I’d download it by dragging out of the original webpage, I’d get a WebP. This was a total WTF for me.

I turned to my friend Jeff Johnson, author of, among other things, the excellent Safari extension StopTheMadness. Not only was Johnson able to explain what was going on, he actually made a new Safari extension called ChangeTheHeaders that fixed the problem for me. Johnson, announcing ChangeTheHeaders last year:

After some investigation, I discovered that the difference was the Accept HTTP request header, which specifies what types of response the web browser will accept. Safari’s default Accept header for images is this:

Accept: image/webp,image/avif,image/jxl,image/heic,image/heic-sequence,video/*;q=0.8,image/png,image/svg+xml,image/*;q=0.8,*/*;q=0.5

Although image/webp appears first in the list, the order actually doesn’t matter. The quality value, specified by the ;q= suffix, determines the ranking of types. The range of values is 0 to 1, with 1 as the default value if none is specified. Thus, image/webp and image/png have equal precedence, equal quality value 1, leaving it up to the web site to decide which image type to serve. In this case, the web site decided to serve a WebP image, despite the fact that the image URL has a .png suffix. In a URL, unlike in a file path, the “file extension”, if one exists, is largely meaningless. A very simple web server will directly match a URL with a local file path, but a more complex web server can do almost anything it wants with a URL.

This was driving me nuts. Thanks to Johnson, I now understand why it was happening, and I had a simple set-it-and-forget-it tool to fix it. Johnson writes:

What can you do with ChangeTheHeaders? I suspect the biggest selling point will be to spoof the User-Agent. The extension allows you to customize your User-Agent by URL domain. For example, you can make Safari pretend that it’s Chrome on Google web apps that give special treatment to Chrome. You can also customize the Accept-Language header if you don’t like the default language handling of some website, such as YouTube.

Here’s the custom rule I applied a year ago, when I first installed ChangeTheHeaders (screenshot):

Header: Accept
Value: image/avif,image/jxl,image/heic,image/heic-sequence,video/*;q=0.8,image/png,image/svg+xml,image/*;q=0.8,*/*;q=0.5
URL Domains: «leave blank for all domains»
URL Filter: «leave blank for all URLs»
Resource Types: image

I haven’t seen a single WebP since.

ChangeTheHeaders works everywhere Safari does — Mac, iPhone, iPad, Vision Pro — and you can get it for just $7 on the App Store.

★ Monday, 2 March 2026