This is caused by a bug in magento 2.3.0, which will be fixed in 2.3.1 release.
You obviously cannot wait until then and you definitely don't want to edit core files.
ref: https://github.com/magento/magento2/issues/18688
If you use composer to install/update magento 2.3.0 (if you don't, you should), you can apply a patch to the code via composer.
The process is explained here:
https://support.magento.com/hc/en-us/articles/360005484154-Create-a-patch-for-a-Magento-2-Composer-installation-from-a-GitHub-commit
For completeness, here is the patch that is needed (edited as noted in the before mentioned link)
diff --git a/view/adminhtml/ui_component/design_config_form.xml b/view/adminhtml/ui_component/design_config_form.xml
index 8d4580f90c7b..bc1f36222dd6 100644
--- a/view/adminhtml/ui_component/design_config_form.xml
+++ b/view/adminhtml/ui_component/design_config_form.xml
@@ -54,7 +54,7 @@
<collapsible>true</collapsible>
<label translate="true">HTML Head</label>
</settings>
- <field name="head_shortcut_icon" formElement="fileUploader">
+ <field name="head_shortcut_icon" formElement="imageUploader">
<settings>
<notice translate="true">Not all browsers support all these formats!</notice>
<label translate="true">Favicon Icon</label>
@@ -151,7 +151,7 @@
<collapsible>true</collapsible>
<label translate="true">Header</label>
</settings>
- <field name="header_logo_src" formElement="fileUploader">
+ <field name="header_logo_src" formElement="imageUploader">
<settings>
<label translate="true">Logo Image</label>
<componentType>imageUploader</componentType>
You want to place this in a folder <magento root>/patches/composer
(I named it : image-upload.diff
)
and then adjust coposer.json accordingly
"extra": {
"magento-force": "override",
"composer-exit-on-patch-failure": true,
"patches": {
"magento/module-theme": {
"MAGETWO-18688: Header logo image upload fix": "patches/composer/image-upload.diff"
}
}
}
The will ensure that the fix is not wiped when you deploy/update etc