platform: extension filter on Windows/macOS system pickers

Windows: IFileOpenDialog SetFileTypes with the allowed extensions
(plus a default extension) when the host passes any.
macOS: NSOpenPanel setAllowedFileTypes from the same list (empty = all).

crates/gpui_macos/src/platform.rs

@@ -13,7 +13,8 @@ use cocoa::{
 	},
 	base::{BOOL, NO, YES, id, nil, selector},
 	foundation::{
-		NSArray, NSAutoreleasePool, NSBundle, NSInteger, NSProcessInfo, NSString, NSUInteger, NSURL,
+		NSArray, NSAutoreleasePool, NSBundle, NSInteger, NSMutableArray, NSProcessInfo, NSString,
+		NSUInteger, NSURL,
 	},
 };
 use core_foundation::{
@@ -767,6 +768,20 @@ impl Platform for MacPlatform {
 						let _: () = msg_send![panel, setPrompt: ns_string(&prompt)];
 					}
 
+					// Extension filter (the host's allowed_extensions): the
+					// open panel's allowedFileTypes. Empty = any.
+					if !options.allowed_extensions.is_empty() {
+						let ns_array = {
+							let arr = NSMutableArray::<NSString>::new();
+							for ext in &options.allowed_extensions {
+								let s = ns_string(ext.as_str());
+								arr.addObject(&*s);
+							}
+							arr
+						};
+						let _: () = msg_send![panel, setAllowedFileTypes: &*ns_array];
+					}
+
 					let _: () = msg_send![panel, beginWithCompletionHandler: block];
 				}
 			})

crates/gpui_windows/src/platform.rs

@@ -1188,6 +1188,30 @@ fn file_open_dialog(
 			folder_dialog.SetOkButtonLabel(&HSTRING::from(prompt))?;
 		}
 
+		// Extension filter (the host's allowed_extensions, e.g.
+		// .ove/.otio/.fcpxml for the project import): the common
+		// ComDlg filter spec.
+		if !options.allowed_extensions.is_empty() {
+			let filter = options
+				.allowed_extensions
+				.iter()
+				.map(|ext| format!("*.{ext}"))
+				.collect::<Vec<_>>()
+				.join(";");
+			// The HSTRINGs must outlive the call (the filter spec holds
+			// raw pointers into them).
+			let name = windows::core::HSTRING::from("Supported files");
+			let pattern = windows::core::HSTRING::from(filter);
+			let spec = Common::COMDLG_FILTERSPEC {
+				pszName: name.as_ptr(),
+				pszSpec: pattern.as_ptr(),
+			};
+			folder_dialog.SetFileTypes(1, &spec)?;
+			folder_dialog.SetDefaultExtension(
+				&HSTRING::from(options.allowed_extensions[0].as_str()),
+			)?;
+		}
+
 		if folder_dialog.Show(window).is_err() {
 			// User cancelled
 			return Ok(None);