(core) move home server into core

Summary: This moves enough server material into core to run a home server.  The data engine is not yet incorporated (though in manual testing it works when ported).

Test Plan: existing tests pass

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2552

app/server/lib/guessExt.ts

@@ -0,0 +1,51 @@
+import {fromFile} from 'file-type';
+import {extension, lookup} from 'mime-types';
+import * as path from 'path';
+
+/**
+ * Get our best guess of the file extension, based on its original extension (as received from the
+ * user), mimeType (as reported by the browser upload, or perhaps some API), and the file
+ * contents.
+ *
+ * The resulting extension is used to choose a parser for imports, and to present the file back
+ * to the user for attachments.
+ */
+export async function guessExt(filePath: string, fileName: string, mimeType: string|null): Promise<string> {
+  const origExt = path.extname(fileName).toLowerCase();   // Has the form ".xls"
+
+  let mimeExt = extension(mimeType);          // Has the form "xls"
+  mimeExt = mimeExt ? "." + mimeExt : null;   // Use the more comparable form ".xls"
+  if (mimeExt === ".json") {
+    // It's common for JSON APIs to specify MIME type, but origExt might come from a URL with
+    // periods that don't indicate a meaningful extension. Trust mime-type here.
+    return mimeExt;
+  }
+
+  if (origExt === ".csv" || origExt === ".xls") {
+    // File type detection doesn't work for these, and mime type can't be trusted. E.g. Windows
+    // may report "application/vnd.ms-excel" for .csv files. See
+    // https://github.com/ManifoldScholar/manifold/issues/2409#issuecomment-545152220
+    return origExt;
+  }
+
+  // If extension and mime type agree, let's call it a day.
+  if (origExt && (origExt === mimeExt || lookup(origExt.slice(1)) === mimeType)) {
+    return origExt;
+  }
+
+  // If not, let's take a look at the file contents.
+  const detected = await fromFile(filePath);
+  const detectedExt = detected ? "." + detected.ext : null;
+  if (detectedExt) {
+    // For the types for which detection works, we think we should prefer it.
+    return detectedExt;
+  }
+
+  if (mimeExt === '.txt' || mimeExt === '.bin') {
+    // text/plain (txt) and application/octet-stream (bin) are too generic, only use them if we
+    // don't have anything better.
+    return origExt || mimeExt;
+  }
+  // In other cases, it's a tough call.
+  return origExt || mimeExt;
+}