[FIX] web_editor, website: use dataset mimetype when modifying images

Since [1] the mimetype used when calling `modify_image` was obtained
from the data URL contained in the `src` attribute instead of the
dataset. This is wrong for background images because they do not have an
`src` attribute.
Thanks to [2] which repaired the value contained in `dataset.mimetype`,
this value can be used as it was before [1].

[1]: https://github.com/odoo/odoo/commit/0449fe85cb0e1d639a4e1aeba26e90906f79254d
[2]: https://github.com/odoo/odoo/commit/40811e72c50faf146ccdf9f26c332db9ebc69fa9

task-3446438

closes odoo/odoo#130322

X-original-commit: e3221fe5d5efa824365210f040e93654e0a30309
Signed-off-by: Romain Derie (rde) <rde@odoo.com>
Signed-off-by: Benoit Socias (bso) <bso@odoo.com>

addons/web_editor/static/src/js/wysiwyg/wysiwyg.js

@@ -2960,7 +2960,7 @@ export class Wysiwyg extends Component {
                 res_id: parseInt(resId),
                 data: (isBackground ? el.dataset.bgSrc : el.getAttribute('src')).split(',')[1],
                 alt_data: altData,
-                mimetype: el.getAttribute('src').split(":")[1].split(";")[0],
+                mimetype: el.dataset.mimetype,
                 name: (el.dataset.fileName ? el.dataset.fileName : null),
             },
         );

addons/website/static/tests/tours/snippet_background_edition.js

@@ -99,6 +99,29 @@ wTourUtils.registerWebsitePreviewTour('snippet_background_edition', {
 wTourUtils.dragNDrop(snippets[0]),
 wTourUtils.clickOnSnippet(snippets[0]),
 
+// Set background image and save.
+{
+    content: "Click on camera icon",
+    trigger: ".snippet-option-ColoredLevelBackground we-button.fa-camera",
+},
+{
+    content: "Click on image",
+    trigger: ".o_select_media_dialog img[title='test.png']",
+},
+...wTourUtils.clickOnSave(),
+{
+    content: "Check that the image is set",
+    trigger: `iframe section.${snippets[0].id} img[data-original-id]`,
+    isCheck: true,
+},
+...wTourUtils.clickOnEditAndWaitEditMode(),
+wTourUtils.clickOnSnippet(snippets[0]),
+// Remove background image.
+{
+    content: "Click on camera icon",
+    trigger: ".snippet-option-ColoredLevelBackground we-button.fa-camera",
+},
+
 // Add a color combination
 ...checkAndUpdateBackgroundColor({
     changeType: 'cc',