fix(frontend): handlePaste deduplication and MIME extension sanitization

[smeinecke]

As I checked #208, I found some other bugs I introduced.

Bug 1: handlePaste double-adds pasted files

In handlePaste, the code first pushes all entries from data.files into raw[], then iterates data.items and pushes every kind === 'file' item — but per spec these represent the same files. The name|size dedup on line 86 hides this in most cases, but two intentionally identical files get silently merged into one.

Fix: iterate only data.items (filtering kind === 'file') and skip the data.files loop entirely.

Bug 2: MIME subtype used verbatim as file extension

const ext = file.name.includes('.') ? '' : `.${file.type.split('/')[1] || 'bin'}`

For image/svg+xml this produces .svg+xml, and for .docx it produces .vnd.openxmlformats-officedocument.... Recipients download files with broken extensions.

Fix: add a mimeToExt() helper with a lookup map for compound MIME types (image/svg+xml → svg, etc.) and strip +suffix from the subtype fallback.

[cupcakearmy]

Thanks for the mime stuff!

I would like to use the mime package and not maintain a custom list :)

mime.getExtension('application/json');         // ⇨ 'json'

Regarding the duplication: i see where this is coming from, but cannot reproduce in ff and/or chrome. How can I repro?