Offer image/webp content type in .desktop file

Nautilus will only offer to open WebP files in applications that accept the `image/webp` content type, not `image/x-webp`.  Even though it isn’t on the [official list](https://www.iana.org/assignments/media-types/media-types.xhtml#image), `image/webp` seems to be more [widely](https://en.wikipedia.org/wiki/WebP) [recommended](https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/MIME_types#Image_types) than `image/x-webp`.
12 jobs for image-webp in 2 minutes and 18 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Prepare
failed #807255
build-image

00:00:16

failed #807257
build-image-win32

00:00:15

failed #807256
build-image-win64

00:00:18

 
  Dependencies
skipped #807258
deps-debian/testing
skipped #807260
deps-win32
skipped #807259
deps-win64
 
  Gimp
skipped #807261
gimp-debian/testing-autotools
skipped #807263
gimp-debian/testing-clang
skipped #807262
gimp-debian/testing-meson
skipped #807265
gimp-win32
skipped #807264
gimp-win64
 
  Analysis
passed #807266
cppcheck

00:02:18

 
Name Stage Failure
failed
build-image-win64 Prepare
$ echo "FROM debian:testing" > Dockerfile
$ echo "RUN apt-get update" >> Dockerfile
$ echo "RUN apt-get install -y --no-install-recommends \\" >> Dockerfile
$ echo "build-essential cpio g++-mingw-w64-x86-64 gcc-mingw-w64-x86-64 git intltool libgdk-pixbuf2.0-dev libglib2.0-bin meson pkg-config python3 python3-distutils python3-docutils python3-pip rpm xsltproc" >> Dockerfile
$ /kaniko/executor --context $CI_PROJECT_DIR --dockerfile $CI_PROJECT_DIR/Dockerfile --destination $CI_REGISTRY_IMAGE:build-win64-latest --cache=true --cache-ttl=120h
error checking push permissions -- make sure you entered the correct tag name, and that you are authenticated correctly, and try again: getting tag for destination: a repository name must be specified
Running after_script
Uploading artifacts for failed job
ERROR: Job failed: exit code 1
failed
build-image-win32 Prepare
$ echo "FROM debian:testing" > Dockerfile
$ echo "RUN apt-get update" >> Dockerfile
$ echo "RUN apt-get install -y --no-install-recommends \\" >> Dockerfile
$ echo "build-essential cpio g++-mingw-w64-i686 gcc-mingw-w64-i686 git intltool libgdk-pixbuf2.0-dev libglib2.0-bin meson pkg-config python3 python3-distutils python3-docutils python3-pip rpm xsltproc" >> Dockerfile
$ /kaniko/executor --context $CI_PROJECT_DIR --dockerfile $CI_PROJECT_DIR/Dockerfile --destination $CI_REGISTRY_IMAGE:build-win32-latest --cache=true --cache-ttl=120h
error checking push permissions -- make sure you entered the correct tag name, and that you are authenticated correctly, and try again: getting tag for destination: a repository name must be specified
Running after_script
Uploading artifacts for failed job
ERROR: Job failed: exit code 1
failed
build-image Prepare
$ echo "FROM debian:testing" > Dockerfile
$ echo "RUN apt-get update" >> Dockerfile
$ echo "RUN apt-get install -y --no-install-recommends \\" >> Dockerfile
$ echo "at-spi2-core build-essential desktop-file-utils ffmpeg ghostscript git glib-networking gobject-introspection graphviz-dev gtk-doc-tools hicolor-icon-theme intltool iso-codes libappstream-glib-dev libbz2-dev libdbus-glib-1-dev libexif-dev libgexiv2-dev libgirepository1.0-dev libgtk-3-bin libgtk-3-dev libgudev-1.0-dev libjson-glib-dev liblcms2-dev liblzma-dev libmng-dev libmypaint-dev libopenexr-dev libpoppler-glib-dev libraw-dev libraw19 librsvg2-dev libspiro-dev libsuitesparse-dev libtiff-dev libtiff5-dev libtool libumfpack5 libwebp-dev libwmf-dev libxmu-dev libxpm-dev luajit meson mypaint-brushes poppler-data python3 valac xauth xvfb" >> Dockerfile
$ /kaniko/executor --context $CI_PROJECT_DIR --dockerfile $CI_PROJECT_DIR/Dockerfile --destination $CI_REGISTRY_IMAGE:build-debian-latest --cache=true --cache-ttl=120h
error checking push permissions -- make sure you entered the correct tag name, and that you are authenticated correctly, and try again: getting tag for destination: a repository name must be specified
Running after_script
Uploading artifacts for failed job
ERROR: Job failed: exit code 1