Fix _g_generic_content_type_get_mime_type to drop the extended part

If the caller wants a MIME/type, assume that it wants just the part
before semicolon (technically, it's not incorrect for MIME/types to
have that, but most of the callers do not expect this).
13 jobs for xdgmime3 in 24 minutes and 44 seconds (queued for 5 seconds)
latest
Status Job ID Name Coverage
  Build
passed #268688
cross-android_api21_arm64

00:01:40

passed #268687
cross-android_api28_arm64

00:01:39

passed #268686
cross-mingw64

00:02:29

passed #268683
fedora-x86_64

00:04:36

passed #268501
G_DISABLE_ASSERT

00:02:49

passed #268505
win32
msys2-mingw32

00:12:16

passed #268506
win32
vs2017-x64

00:08:54

failed #268502
cross-android_api21_arm64

00:00:24

failed #268503
cross-android_api28_arm64

00:00:27

failed #268504
cross-mingw64

00:00:26

failed #268677
fedora-x86_64

00:04:24

failed #268500
fedora-x86_64

00:00:22

 
  Coverage
passed #268507
coverage

00:02:02

74.7%