diff --git a/Apps/MagnumOpus/ViewModels/ComposeViewModel.swift b/Apps/MagnumOpus/ViewModels/ComposeViewModel.swift
index 49fd71f..410b003 100644
--- a/Apps/MagnumOpus/ViewModels/ComposeViewModel.swift
+++ b/Apps/MagnumOpus/ViewModels/ComposeViewModel.swift
@@ -1,9 +1,19 @@
 import SwiftUI
+import UniformTypeIdentifiers
 import Models
 import MailStore
 import SyncEngine
 import SMTPClient
 
+struct ComposeAttachment: Identifiable {
+	let id = UUID()
+	let url: URL
+	let filename: String
+	let mimeType: String
+	let size: Int
+	let data: Data
+}
+
 enum ComposeMode: Sendable {
 	case new
 	case reply(to: MessageSummary)
@@ -21,6 +31,9 @@ final class ComposeViewModel {
 	var subject: String = ""
 	var bodyText: String = ""
 
+	var attachments: [ComposeAttachment] = []
+	var sizeWarningMessage: String?
+
 	var isSending: Bool = false
 	var errorMessage: String?
 
@@ -59,6 +72,35 @@ final class ComposeViewModel {
 		// since it holds a weak reference to self
 	}
 
+	// MARK: - Attachments
+
+	func addAttachment(url: URL) {
+		do {
+			let data = try Data(contentsOf: url)
+			let size = data.count
+			let maxSize = 25 * 1024 * 1024
+			let warnSize = 10 * 1024 * 1024
+
+			if size > maxSize {
+				errorMessage = "\(url.lastPathComponent) exceeds 25 MB limit (\(ByteCountFormatter.string(fromByteCount: Int64(size), countStyle: .file)))."
+				return
+			}
+			if size > warnSize {
+				sizeWarningMessage = "\(url.lastPathComponent) is \(ByteCountFormatter.string(fromByteCount: Int64(size), countStyle: .file)). Large attachments may slow delivery."
+			}
+
+			let ext = url.pathExtension
+			let mimeType = UTType(filenameExtension: ext)?.preferredMIMEType ?? "application/octet-stream"
+			attachments.append(ComposeAttachment(url: url, filename: url.lastPathComponent, mimeType: mimeType, size: size, data: data))
+		} catch {
+			errorMessage = "Could not read file: \(error.localizedDescription)"
+		}
+	}
+
+	func removeAttachment(id: UUID) {
+		attachments.removeAll { $0.id == id }
+	}
+
 	// MARK: - Send
 
 	func send() async throws {
@@ -104,10 +146,19 @@ final class ComposeViewModel {
 				bodyText: bodyText,
 				inReplyTo: inReplyTo,
 				references: references,
-				messageId: messageId
+				messageId: messageId,
+				attachments: attachments.map { OutgoingAttachment(filename: $0.filename, mimeType: $0.mimeType, data: $0.data) }
 			)
 
-			let formatted = MessageFormatter.format(outgoing)
+			let formatted: String
+			if attachments.isEmpty {
+				formatted = MessageFormatter.format(outgoing)
+			} else {
+				formatted = try MessageFormatter.formatMultipart(
+					outgoing,
+					attachments: attachments.map { ($0.filename, $0.mimeType, $0.data) }
+				)
+			}
 
 			let sendAction = PendingAction(
 				accountId: accountConfig.id,
diff --git a/Apps/MagnumOpus/Views/ComposeView.swift b/Apps/MagnumOpus/Views/ComposeView.swift
index 6ce1a18..772d32d 100644
--- a/Apps/MagnumOpus/Views/ComposeView.swift
+++ b/Apps/MagnumOpus/Views/ComposeView.swift
@@ -1,4 +1,5 @@
 import SwiftUI
+import UniformTypeIdentifiers
 import Models
 
 struct ComposeView: View {
@@ -7,6 +8,7 @@ struct ComposeView: View {
 
 	@State private var showBcc = false
 	@State private var showDiscardConfirmation = false
+	@State private var showFileImporter = false
 
 	var body: some View {
 		#if os(macOS)
@@ -49,8 +51,50 @@ struct ComposeView: View {
 			}
 
 			Section {
+				#if os(macOS)
+				PastableTextEditor(text: $viewModel.bodyText) { url in
+					viewModel.addAttachment(url: url)
+				}
+				.frame(minHeight: 200)
+				.onDrop(of: [.fileURL], isTargeted: nil) { providers in
+					for provider in providers {
+						_ = provider.loadObject(ofClass: URL.self) { url, _ in
+							if let url {
+								Task { @MainActor in
+									viewModel.addAttachment(url: url)
+								}
+							}
+						}
+					}
+					return true
+				}
+				#else
 				TextEditor(text: $viewModel.bodyText)
 					.frame(minHeight: 200)
+				#endif
+			}
+
+			if !viewModel.attachments.isEmpty {
+				Section {
+					ScrollView(.horizontal, showsIndicators: false) {
+						HStack(spacing: 8) {
+							ForEach(viewModel.attachments) { attachment in
+								AttachmentChip(attachment: attachment) {
+									viewModel.removeAttachment(id: attachment.id)
+								}
+							}
+						}
+						.padding(.vertical, 4)
+					}
+				}
+			}
+
+			if let warning = viewModel.sizeWarningMessage {
+				Section {
+					Text(warning)
+						.foregroundStyle(.orange)
+						.font(.caption)
+				}
 			}
 
 			if let error = viewModel.errorMessage {
@@ -60,6 +104,18 @@ struct ComposeView: View {
 				}
 			}
 		}
+		.fileImporter(isPresented: $showFileImporter, allowedContentTypes: [.item], allowsMultipleSelection: true) { result in
+			switch result {
+			case .success(let urls):
+				for url in urls {
+					guard url.startAccessingSecurityScopedResource() else { continue }
+					defer { url.stopAccessingSecurityScopedResource() }
+					viewModel.addAttachment(url: url)
+				}
+			case .failure(let error):
+				viewModel.errorMessage = error.localizedDescription
+			}
+		}
 		.formStyle(.grouped)
 		.navigationTitle(navigationTitle)
 		#if os(macOS)
@@ -86,6 +142,16 @@ struct ComposeView: View {
 				.help("Toggle BCC field")
 			}
 
+			ToolbarItem(placement: .automatic) {
+				Button {
+					showFileImporter = true
+				} label: {
+					Label("Attach", systemImage: "paperclip")
+				}
+				.keyboardShortcut("a", modifiers: [.shift, .command])
+				.help("Attach File (⇧⌘A)")
+			}
+
 			ToolbarItem(placement: .confirmationAction) {
 				Button {
 					Task {
@@ -136,3 +202,119 @@ struct ComposeView: View {
 		}
 	}
 }
+
+
+struct AttachmentChip: View {
+	let attachment: ComposeAttachment
+	let onRemove: () -> Void
+
+	var body: some View {
+		HStack(spacing: 4) {
+			Image(systemName: iconName)
+				.foregroundStyle(.secondary)
+			Text(attachment.filename)
+				.lineLimit(1)
+				.truncationMode(.middle)
+				.frame(maxWidth: 150)
+			Text("·")
+				.foregroundStyle(.tertiary)
+			Text(ByteCountFormatter.string(fromByteCount: Int64(attachment.size), countStyle: .file))
+				.foregroundStyle(.secondary)
+			Button {
+				onRemove()
+			} label: {
+				Image(systemName: "xmark.circle.fill")
+					.foregroundStyle(.secondary)
+			}
+			.buttonStyle(.plain)
+		}
+		.font(.caption)
+		.padding(.horizontal, 8)
+		.padding(.vertical, 4)
+		.background(.quaternary, in: Capsule())
+	}
+
+	private var iconName: String {
+		let ext = (attachment.filename as NSString).pathExtension.lowercased()
+		switch ext {
+		case "jpg", "jpeg", "png", "gif", "heic", "webp": return "photo"
+		case "pdf": return "doc.richtext"
+		case "mp4", "mov", "avi": return "film"
+		case "mp3", "wav", "aac", "m4a": return "music.note"
+		case "zip", "gz", "tar", "7z": return "doc.zipper"
+		default: return "paperclip"
+		}
+	}
+}
+
+#if os(macOS)
+import AppKit
+
+struct PastableTextEditor: NSViewRepresentable {
+	@Binding var text: String
+	var onPasteFile: (URL) -> Void
+
+	func makeNSView(context: Context) -> NSScrollView {
+		let scrollView = NSTextView.scrollableTextView()
+		let textView = scrollView.documentView as! NSTextView
+		textView.delegate = context.coordinator
+		textView.font = .systemFont(ofSize: 14)
+		textView.isRichText = false
+		textView.allowsUndo = true
+		textView.string = text
+		context.coordinator.textView = textView
+		return scrollView
+	}
+
+	func updateNSView(_ nsView: NSScrollView, context: Context) {
+		let textView = nsView.documentView as! NSTextView
+		if textView.string != text {
+			textView.string = text
+		}
+	}
+
+	func makeCoordinator() -> Coordinator {
+		Coordinator(text: $text, onPasteFile: onPasteFile)
+	}
+
+	class Coordinator: NSObject, NSTextViewDelegate {
+		@Binding var text: String
+		var onPasteFile: (URL) -> Void
+		weak var textView: NSTextView?
+
+		init(text: Binding<String>, onPasteFile: @escaping (URL) -> Void) {
+			_text = text
+			self.onPasteFile = onPasteFile
+		}
+
+		func textDidChange(_ notification: Notification) {
+			guard let textView else { return }
+			text = textView.string
+		}
+
+		func textView(_ textView: NSTextView, doCommandBy commandSelector: Selector) -> Bool {
+			if commandSelector == #selector(NSText.paste(_:)) {
+				let pasteboard = NSPasteboard.general
+				if let urls = pasteboard.readObjects(forClasses: [NSURL.self]) as? [URL] {
+					for url in urls where url.isFileURL {
+						onPasteFile(url)
+					}
+					if !urls.isEmpty { return true }
+				}
+				if let images = pasteboard.readObjects(forClasses: [NSImage.self]) as? [NSImage],
+				   let image = images.first,
+				   let tiffData = image.tiffRepresentation,
+				   let bitmap = NSBitmapImageRep(data: tiffData),
+				   let pngData = bitmap.representation(using: .png, properties: [:]) {
+					let tempFile = FileManager.default.temporaryDirectory
+						.appendingPathComponent("pasted-image-\(UUID().uuidString).png")
+					try? pngData.write(to: tempFile)
+					onPasteFile(tempFile)
+					return true
+				}
+			}
+			return false
+		}
+	}
+}
+#endif