help: Add mobile instructions to "Share and upload files".

- Documents uploading files via compose box buttons.
- Documents sharing files from other Android apps.

help/share-and-upload-files.md

@@ -23,6 +23,11 @@ and you can click on a thumbnail to [view the full image](/help/view-and-browse-
 
 1. _(optional)_ Modify the link text as desired.
 
+!!! tip ""
+
+    You can [preview the message](/help/preview-your-message-before-sending)
+    before sending to see what your uploaded files will look like.
+
 {tab|via-compose-box-buttons}
 
 {!start-composing.md!}
@@ -35,13 +40,53 @@ and you can click on a thumbnail to [view the full image](/help/view-and-browse-
 
 1. _(optional)_ Modify the link text as desired.
 
-{end_tabs}
-
 !!! tip ""
 
     You can [preview the message](/help/preview-your-message-before-sending)
     before sending to see what your uploaded files will look like.
 
+{tab|mobile}
+
+1. Navigate to a stream, topic, or direct message view.
+
+1. Tap the
+   **paperclip** (<img src="/static/images/help/mobile-paperclip-icon.svg" alt="paperclip" class="mobile-icon"/>),
+   **image** (<img src="/static/images/help/mobile-image-icon.svg" alt="image" class="mobile-icon"/>),
+   or **camera** (<img src="/static/images/help/mobile-camera-icon.svg" alt="camera" class="mobile-icon"/>)
+   button at the bottom of the app to select one or more files. Zulip will
+   upload the files, and insert named links using
+   [Markdown formatting](/help/format-your-message-using-markdown#links):
+   `[Link text](URL)`.
+
+1. _(optional)_ Modify the link text as desired.
+
+{end_tabs}
+
+!!! tip ""
+
+    The link text will default to the name of the uploaded file.
+
+## Sharing files
+
+You can share files from other apps on Zulip.
+
+{start_tabs}
+
+{tab|android}
+
+1. Select one or more files and tap the **Zulip**
+   (<img src="/static/images/logo/zulip-icon-circle.svg" alt="logo" class="mobile-icon"/>)
+   logo.
+
+1. Select a stream name and topic name, or tap the
+   **Direct message** tab and **Choose recipients**.
+
+1. _(optional)_ Write a message.
+
+1. Tap the **Send** button.
+
+{end_tabs}
+
 ## Named file example
 
 ### What you type