diff --git a/addons/mail/static/src/components/composer/composer.js b/addons/mail/static/src/components/composer/composer.js
index 5c3968a0fae..afb7d811eae 100644
--- a/addons/mail/static/src/components/composer/composer.js
+++ b/addons/mail/static/src/components/composer/composer.js
@@ -175,9 +175,21 @@ class Composer extends Component {
     /**
      * Post a message in the composer on related thread.
      *
+     * Posting of the message could be aborted if it cannot be posted like if there are attachments
+     * currently uploading or if there is no text content and no attachments.
+     *
      * @private
      */
     async _postMessage() {
+        if (!this.composer.canPostMessage) {
+            if (this.composer.hasUploadingAttachment) {
+                this.env.services['notification'].notify({
+                    message: this.env._t("Please wait while the file is uploading."),
+                    type: 'warning',
+                });
+            }
+            return;
+        }
         // TODO: take suggested recipients into account (task-2283356)
         await this.composer.postMessage();
         // TODO: we might need to remove trigger and use the store to wait for the post rpc to be done
@@ -246,9 +258,6 @@ class Composer extends Component {
      * @private
      */
     _onClickSend() {
-        if (!this.composer.canPostMessage) {
-            return;
-        }
         this._postMessage();
     }
 
@@ -335,9 +344,6 @@ class Composer extends Component {
      * @private
      */
     _onTextInputKeydownEnter() {
-        if (!this.composer.canPostMessage) {
-            return;
-        }
         this._postMessage();
     }
 
diff --git a/addons/mail/static/src/components/composer/composer.xml b/addons/mail/static/src/components/composer/composer.xml
index 5a7a2e72345..30053cdd0f8 100644
--- a/addons/mail/static/src/components/composer/composer.xml
+++ b/addons/mail/static/src/components/composer/composer.xml
@@ -155,7 +155,7 @@
                         'o-composer-is-compact': props.isCompact,
                         'o-has-current-partner-avatar': props.hasCurrentPartnerAvatar,
                     }"
-                    t-att-disabled="(!composer.textInputContent and composer.attachments.length === 0) ? 'disabled' : ''"
+                    t-att-disabled="!composer.canPostMessage ? 'disabled' : ''"
                     type="button"
                     t-on-click="_onClickSend"
                 >
diff --git a/addons/mail/static/src/components/composer/composer_tests.js b/addons/mail/static/src/components/composer/composer_tests.js
index 527003b6dec..8e07affcccb 100644
--- a/addons/mail/static/src/components/composer/composer_tests.js
+++ b/addons/mail/static/src/components/composer/composer_tests.js
@@ -21,6 +21,7 @@ const {
         createFile,
         inputFiles,
     },
+    makeTestPromise,
 } = require('web.test_utils');
 
 QUnit.module('mail', {}, function () {
@@ -886,6 +887,148 @@ QUnit.test('current partner notify is typing again to other members every 50s of
     );
 });
 
+QUnit.test('composer: send button is disabled if attachment upload is not finished', async function (assert) {
+    assert.expect(8);
+
+    const attachmentUploadedPromise = makeTestPromise();
+    await this.start({
+        async mockFetch(resource, init) {
+            const res = this._super(...arguments);
+            if (resource === '/web/binary/upload_attachment') {
+                await attachmentUploadedPromise;
+            }
+            return res;
+        }
+    });
+    const composer = this.env.models['mail.composer'].create();
+    await this.createComposerComponent(composer);
+    const file = await createFile({
+        content: 'hello, world',
+        contentType: 'text/plain',
+        name: 'text.txt',
+    });
+    await afterNextRender(() =>
+        inputFiles(
+            document.querySelector('.o_FileUploader_input'),
+            [file]
+        )
+    );
+    assert.containsOnce(
+        document.body,
+        '.o_Attachment',
+        "should have an attachment after a file has been input"
+    );
+    assert.containsOnce(
+        document.body,
+        '.o_Attachment.o-temporary',
+        "attachment displayed is being uploaded"
+    );
+    assert.containsOnce(
+        document.body,
+        '.o_Composer_buttonSend',
+        "composer send button should be displayed"
+    );
+    assert.ok(
+        !!document.querySelector('.o_Composer_buttonSend').attributes.disabled,
+        "composer send button should be disabled as attachment is not yet uploaded"
+    );
+
+    // simulates attachment finishes uploading
+    await afterNextRender(() => attachmentUploadedPromise.resolve());
+    assert.containsOnce(
+        document.body,
+        '.o_Attachment',
+        "should have only one attachment"
+    );
+    assert.containsNone(
+        document.body,
+        '.o_Attachment.o-temporary',
+        "attachment displayed should be uploaded"
+    );
+    assert.containsOnce(
+        document.body,
+        '.o_Composer_buttonSend',
+        "composer send button should still be present"
+    );
+    assert.ok(
+        !document.querySelector('.o_Composer_buttonSend').attributes.disabled,
+        "composer send button should be enabled as attachment is now uploaded"
+    );
+});
+
+QUnit.test('warning when attempting to post message when attachments are still uploading', async function (assert) {
+    assert.expect(7);
+
+    await this.start({
+        async mockFetch(resource, init) {
+            const res = this._super(...arguments);
+            if (resource === '/web/binary/upload_attachment') {
+                // simulates attachment is never finished uploading
+                await new Promise(() => {});
+            }
+            return res;
+        },
+        services: {
+            notification: {
+                notify(params) {
+                    assert.strictEqual(
+                        params.message,
+                        "Please wait while the file is uploading.",
+                        "notification content should be about the uploading file"
+                    );
+                    assert.strictEqual(
+                        params.type,
+                        'warning',
+                        "notification should be a warning"
+                    );
+                    assert.step('notification');
+                }
+            }
+        },
+    });
+    const thread = this.env.models['mail.thread'].create({
+        composer: [['create', { isLog: false }]],
+        id: 20,
+        model: 'res.partner',
+    });
+    await this.createComposerComponent(thread.composer);
+    const file = await createFile({
+        content: 'hello, world',
+        contentType: 'text/plain',
+        name: 'text.txt',
+    });
+    await afterNextRender(() =>
+        inputFiles(
+            document.querySelector('.o_FileUploader_input'),
+            [file]
+        )
+    );
+    assert.containsOnce(
+        document.body,
+        '.o_Attachment',
+        "should have only one attachment"
+    );
+    assert.containsOnce(
+        document.body,
+        '.o_Attachment.o-temporary',
+        "attachment displayed is being uploaded"
+    );
+    assert.containsOnce(
+        document.body,
+        '.o_Composer_buttonSend',
+        "composer send button should be displayed"
+    );
+
+    // Try to send message
+    document
+        .querySelector(`.o_ComposerTextInput_textarea`)
+        .dispatchEvent(new window.KeyboardEvent('keydown', { key: 'Enter' }));
+    assert.verifySteps(
+        ['notification'],
+        "should have triggered a notification for inability to post message at the moment (some attachments are still being uploaded)"
+    );
+});
+
 });
 });
 });
diff --git a/addons/mail/static/src/models/composer/composer.js b/addons/mail/static/src/models/composer/composer.js
index f2d376fd14b..421ca3061d2 100644
--- a/addons/mail/static/src/models/composer/composer.js
+++ b/addons/mail/static/src/models/composer/composer.js
@@ -294,7 +294,10 @@ function factory(dependencies) {
          * @returns {boolean}
          */
         _computeCanPostMessage() {
-            return this.textInputContent || this.attachments.length !== 0;
+            if (!this.textInputContent && this.attachments.length === 0) {
+                return false;
+            }
+            return !this.hasUploadingAttachment;
         }
 
         /**
@@ -319,6 +322,14 @@ function factory(dependencies) {
             return this.allSuggestedPartners.length > 0;
         }
 
+        /**
+         * @private
+         * @returns {boolean}
+         */
+        _computeHasUploadingAttachment() {
+            return this.attachments.some(attachment => attachment.isTemporary);
+        }
+
         /**
          * Detects if mentioned partners are still in the composer text input content
          * and removes them if not.
@@ -540,10 +551,21 @@ function factory(dependencies) {
         attachments: many2many('mail.attachment', {
             inverse: 'composers',
         }),
+        /**
+         * This field watches the uploading (= temporary) status of attachments
+         * linked to this composer.
+         *
+         * Useful to determine whether there are some attachments that are being
+         * uploaded.
+         */
+        attachmentsAreTemporary: attr({
+            related: 'attachments.isTemporary',
+        }),
         canPostMessage: attr({
             compute: '_computeCanPostMessage',
             dependencies: [
                 'attachments',
+                'hasUploadingAttachment',
                 'textInputContent',
             ],
             default: false,
@@ -563,6 +585,17 @@ function factory(dependencies) {
                 'mainSuggestedPartners',
             ],
         }),
+        /**
+         * This field determines whether some attachments linked to this
+         * composer are being uploaded.
+         */
+        hasUploadingAttachment: attr({
+            compute: '_computeHasUploadingAttachment',
+            dependencies: [
+                'attachments',
+                'attachmentsAreTemporary',
+            ],
+        }),
         hasFocus: attr({
             default: false,
         }),
diff --git a/addons/mail/static/src/utils/test_utils.js b/addons/mail/static/src/utils/test_utils.js
index d42c1318dfa..dfad584dfeb 100644
--- a/addons/mail/static/src/utils/test_utils.js
+++ b/addons/mail/static/src/utils/test_utils.js
@@ -368,6 +368,7 @@ async function createRootComponent(self, Component, { props = {}, target }) {
  *   is set: provide data that is passed to discuss widget (= client action) as
  *   2nd positional argument.
  * @param {Object} [param0.env={}]
+ * @param {function} [param0.mockFetch]
  * @param {function} [param0.mockRPC]
  * @param {boolean} [param0.hasActionManager=false] if set, use
  *   createActionManager.
diff --git a/addons/web/static/tests/helpers/test_utils_mock.js b/addons/web/static/tests/helpers/test_utils_mock.js
index 06f9ce33d8a..962b9610f4b 100644
--- a/addons/web/static/tests/helpers/test_utils_mock.js
+++ b/addons/web/static/tests/helpers/test_utils_mock.js
@@ -333,6 +333,7 @@ function removeSrcAttribute(el, rpc) {
  * @param {string} [params.currentDate]
  * @param {Object} [params.data]
  * @param {boolean} [params.debug]
+ * @param {function} [params.mockFetch]
  * @param {function} [params.mockRPC]
  * @param {number} [params.fieldDebounce=0] the value of the DEBOUNCE attribute
  *   of fields
@@ -351,8 +352,11 @@ async function addMockEnvironmentOwl(Component, params, mockServer) {
     // instantiate a mockServer if not provided
     if (!mockServer) {
         let Server = MockServer;
+        if (params.mockFetch) {
+            Server = MockServer.extend({ _performFetch: params.mockFetch });
+        }
         if (params.mockRPC) {
-            Server = MockServer.extend({ _performRpc: params.mockRPC });
+            Server = Server.extend({ _performRpc: params.mockRPC });
         }
         mockServer = new Server(params.data, {
             actions: params.actions,
@@ -466,6 +470,9 @@ async function addMockEnvironmentOwl(Component, params, mockServer) {
  * @param {number} [params.debug] if set to true, logs RPCs and uncaught Odoo
  *   events.
  * @param {Object} [params.bus] the instance of Bus that will be used (in the env)
+ * @param {function} [params.mockFetch] a function that will be used to override
+ *   the _performFetch method from the mock server. It is really useful to add
+ *   some custom fetch mocks, or to check some assertions.
  * @param {function} [params.mockRPC] a function that will be used to override
  *   the _performRpc method from the mock server. It is really useful to add
  *   some custom rpc mocks, or to check some assertions.
@@ -506,8 +513,11 @@ async function addMockEnvironment(widget, params) {
 
     // instantiate mock server
     var Server = MockServer;
+    if (params.mockFetch) {
+        Server = MockServer.extend({ _performFetch: params.mockFetch });
+    }
     if (params.mockRPC) {
-        Server = MockServer.extend({ _performRpc: params.mockRPC });
+        Server = Server.extend({ _performRpc: params.mockRPC });
     }
     var mockServer = new Server(params.data, {
         actions: params.actions,