first upload all files

Modules/Media/Resources/assets/admin/sass/main.scss

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+@import '~dropzone/dist/dropzone';
+
+.dropzone {
+    border: 1px dashed #d2d6de;
+    min-height: 232px;
+    margin-bottom: 20px;
+
+    &.dz-clickable {
+        border-radius: 3px;
+    }
+
+    .dz-message {
+        color: #646C7F;
+        font-size: 20px;
+        margin-top: 82px;
+    }
+
+    .dz-preview .dz-progress .dz-upload {
+        background: #0068e1;
+    }
+}
+
+.file-icon {
+	font-size: 20px;
+}
+
+.main-file {
+    width: 100%;
+}
+
+.file-thumbnail {
+    float: left;
+    margin-right: 10px;
+
+    > .thumbnail-name {
+        text-align: center;
+    }
+}
+
+.media-picker-divider {
+    border-bottom: 1px solid #e9e9e9;
+    margin-bottom: 20px;
+}
+
+.single-image-wrapper {
+    padding-bottom: 20px;
+
+    h4 {
+        font-weight: 500;
+        margin-bottom: 10px;
+    }
+}
+
+.single-image {
+	padding: 10px;
+	background: #f1f1f1;
+	margin-top: 15px;
+    display: inline-block;
+    border-radius: 3px;
+    vertical-align: bottom;
+
+    > .image-holder {
+        cursor: default;
+        margin: 0;
+        border-radius: 3px;
+    }
+}
+
+.multiple-images-wrapper {
+    margin-top: 15px;
+
+    h4 {
+        font-weight: 500;
+        margin-bottom: 10px;
+    }
+}
+
+.multiple-images {
+	overflow: hidden;
+	padding: 10px 10px 0 10px;
+	background: #f1f1f1;
+	margin-top: 15px;
+    border-radius: 3px;
+}
+
+.image-holder {
+	position: relative;
+	float: left;
+	height: 125px;
+	width: 125px;
+	overflow: hidden;
+	background: #fff;
+	margin: 0 10px 10px 0;
+	border: 1px solid #d2d6de;
+    border-radius: 3px;
+	cursor: move;
+    z-index: 0;
+
+    > i {
+        position: absolute;
+        font-size: 60px;
+        color: #d9d9d9;
+        top: 50%;
+        left: 50%;
+        transform: translate(-50%,-50%);
+        z-index: -1;
+    }
+
+    > img {
+        position: absolute;
+        left: 50%;
+        top: 50%;
+        max-height: 100%;
+        max-width: 100%;
+        transform: translate(-50%, -50%);
+        z-index: 1;
+    }
+
+    > .remove-image {
+        position: absolute;
+        top: -2px;
+        right: 4px;
+        color: #ffffff;
+        padding: 0;
+        background: transparent;
+        transition: 200ms;
+        visibility: hidden;
+        opacity: 0;
+        font-size: 18px;
+        font-family: 'FontAwesome';
+        -webkit-text-stroke: 1px #737881;
+           -moz-text-stroke: 1px #737881;
+            -ms-text-stroke: 1px #737881;
+             -o-text-stroke: 1px #737881;
+        z-index: 2;
+
+        &:focus {
+            border-color: transparent;
+            -webkit-box-shadow: none;
+                    box-shadow: none;
+        }
+
+        &:active {
+            -webkit-box-shadow: none;
+                    box-shadow: none;
+        }
+
+        &::after {
+            content: "\f00d";
+        }
+    }
+
+    &:hover > .remove-image {
+        visibility: visible;
+        opacity: 1;
+    }
+}
+
+.image-picker > i {
+    color: #737881;
+}

Modules/Media/Resources/assets/admin/sass/media-picker.scss

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+.file-manager-iframe {
+	width: 100%;
+	vertical-align: bottom;
+}
+
+.file-manager {
+	background: #f9f9f9;
+    margin-top: 20px;
+    overflow-x: auto;
+
+    #notification-toast {
+        bottom: 0;
+    }
+
+    .dataTable {
+        .btn {
+            padding: 10px 16px 8px;
+
+            > i {
+                margin-top: 5px;
+            }
+        }
+    }
+}
+
+.media-picker-modal {
+    padding-right: 0 !important;
+    z-index: 1050;
+
+    > i {
+        margin-right: 5px;
+    }
+
+    .modal-dialog {
+        width: auto;
+        margin: 25px auto;
+    }
+
+    .modal-content {
+        margin: auto;
+        float: none;
+        border: 1px solid #d2d6de;
+        border-radius: 3px;
+    }
+
+    .modal-header {
+        padding: 6px 15px;
+        background: #f1f1f1;
+
+        > .close {
+            margin-top: 5px;
+            -webkit-text-stroke: 0;
+            transition: 200ms;
+        }
+    }
+
+    .modal-body {
+        background: #f9f9f9;
+        padding: 15px 0;
+    }
+
+    .modal.fade .modal-dialog {
+        transform: scale(0.8);
+        opacity: 0;
+        transition: 200ms ease-in-out;
+
+        &.in .modal-dialog {
+            transform: scale(1);
+            opacity: 1;
+        }
+    }
+}
+
+@media screen and (max-width: 767px) {
+	.media-picker-modal .modal-dialog {
+		margin: 10px;
+	}
+}