Ability to add additional images to pending upload queue

### Your use case

#### What would you like to do?
Be able to add more images to the upload queue after already queuing an initial image for upload, rather than being limited to only selecting multiple images simultaneously.
#### Why would you like to do it?
When sharing multiple images in a conversation, it's common to decide to add more images while the first ones are set to upload. Currently, this requires sending the first upload queue in order to start another upload queue, or starting over with a new multi-select, which interrupts the natural flow of conversation.
#### How would you like to achieve it?
Add the ability to queue additional images (via drag & drop or the "Upload files" UI) while other images are still in the upload queue. This could be implemented through:
- An "Add more" button in the upload queue interface
- Allowing drag & drop to append to the existing queue

### Have you considered any alternatives?

Current workarounds are:
1. Selecting all images at once through multi-select in the initial file selection
2. Creating a local folder to gather all images before uploading
3. Sending each upload queue before starting the next upload queue
However, these workarounds are less efficient than proper queue management.

### Additional context

This feature is particularly important for users who frequently share multiple images in conversations, especially when the images are in different folders.