feat: adds drag, drop and pasting of attachments

[steven-mpawulo]

Add support for attaching files via drag-and-drop and clipboard paste in the message composer.

This reuses the existing attachment upload flow to ensure consistent upload handling and validation.

Fixes #12687.

[ChristophWurst]

Thanks for the feature! Code looks good on a high level. We'll test and review in detail soon.

CI is unhappy about the missing sign-off trailer in your commit.

[steven-mpawulo]

Thanks
I'll add the missing sign-off trailer and push an updated commit shortly.
@ChristophWurst

[GretaD]

hi @steven-mpawulo, i tested it and i am able to drag and drop some files.
I noticed a couple of problems though:

I cannot drag and drop pdf files.
When i try to add a big file, the popup that my file extends the upload limit is not shown.

[steven-mpawulo]

Hello @GretaD
Let me look into these issues and resolve them accordingly.

[steven-mpawulo]

@GretaD I'm a bit lost here.

If attachment-size-limit is not set, the default value is 0, which means the popup never appears.
How does the admin currently enforce file size limits?
Should we change the default value to something more reasonable?