Say which input the image processor refuses before decoding

README.md, the doc comment of ErrUnsupportedInputFormat and the TODO.md
entry say the 415 is for input whose first bytes libvips reads as
another format, and no longer read as refusing a damaged image in a
source format, which is decoded and answers 500. The doc comment of
TestImageProcessor_OrigOfUnnamedFormatRefused says TIFF is refused
before it is decoded.

Model: opus-5-5

README.md

@@ -192,9 +192,10 @@ path under `/v1/` answers 200, in maintenance mode too.
   address in a blocked network (see `blocked_networks`); 429 over
   `image_requests_per_minute` (below); 415 when the upstream answers with a
   `Content-Type` that is not one of the source formats (below), or with an image
-  libvips would not load as one of them; 502 when the upstream answered with an
-  error status, and for 5 minutes after that for the same source URL; 503 when
-  pixa is busy or in maintenance mode; 500 for any other failure.
+  whose first bytes libvips reads as another format; 502 when the upstream
+  answered with an error status, and for 5 minutes after that for the same
+  source URL; 503 when pixa is busy or in maintenance mode; 500 for any other
+  failure.
 - `GET` or `HEAD` `/v1/e/<token>/<name>` — an image through an encrypted URL
   (see Encrypted URLs). Needs: nothing but the URL. Answers: 200; 304 when
   `If-None-Match` matches the image's `ETag`; 400 for a token that does not
@@ -265,8 +266,8 @@ once, is refused with 400.
 The source image may be JPEG, PNG, GIF, WebP, AVIF or JPEG XL. The upstream's
 `Content-Type` must name its format, and the image's first bytes must match it.
 An upstream answering with any other `Content-Type`, SVG included, is refused
-with 415, and so is an image that libvips would not load as one of these
-formats, such as an SVG whose first bytes pass the check for AVIF.
+with 415, and so is an image whose first bytes libvips reads as another format,
+such as an SVG whose first bytes pass the check for AVIF.
 
 With the format `auto`, pixa chooses the format for each request from its
 `Accept` header, in this order: