diff --git a/README.md b/README.md
index ae8b1eea..4c0a98d6 100644
--- a/README.md
+++ b/README.md
@@ -408,24 +408,7 @@ const document = await FileAttachment("index.html").html();
 
 *Note: this function is not part of the Observable standard library (in notebooks), but is provided by this module as a means for defining custom file attachment implementations when working directly with the Observable runtime.*
 
-Returns a [*FileAttachment*](#FileAttachment) function given the specified *resolve* function. The *resolve* function is an async function that takes a *name* and returns a URL at which the file of that name can be loaded. For example:
-
-```js
-const FileAttachment = FileAttachments((name) =>
-  `https://my.server/notebooks/demo/${name}`
-);
-```
-
-Or, with a more complex example, calling an API to produce temporary URLs:
-
-```js
-const FileAttachment = FileAttachments(async (name) =>
-  if (cachedUrls.has(name)) return cachedUrls.get(name);
-  const url = await fetchSignedFileUrl(notebookId, name);
-  cachedUrls.set(name, url);
-  return url;
-);
-```
+Returns a [*FileAttachment*](#FileAttachment) function given the specified *resolve* function. The *resolve* function is a function that takes a *name* and returns either an object {url, mimeType} representing the requested file if it exists, or null if the file does not exist. The url field (though not the object itself!) may be represented as a Promise if the URL is not yet known, such as for a file that is currently being uploaded. The mimeType must be a string, or undefined if the mime type is not known. For backwards compatibility, the *resolve* function may instead return just a URL, either a string or a promise.
 
 ### Generators
 
diff --git a/src/fileAttachment.js b/src/fileAttachment.js
index efe2df43..9cb3f88a 100644
--- a/src/fileAttachment.js
+++ b/src/fileAttachment.js
@@ -18,8 +18,9 @@ async function dsv(file, delimiter, {array = false, typed = false} = {}) {
 }
 
 export class AbstractFile {
-  constructor(name) {
+  constructor(name, mimeType) {
     Object.defineProperty(this, "name", {value: name, enumerable: true});
+    if (mimeType !== undefined) Object.defineProperty(this, "mimeType", {value: mimeType + "", enumerable: true});
   }
   async blob() {
     return (await remote_fetch(this)).blob();
@@ -79,8 +80,8 @@ export class AbstractFile {
 }
 
 class FileAttachment extends AbstractFile {
-  constructor(url, name) {
-    super(name);
+  constructor(url, name, mimeType) {
+    super(name, mimeType);
     Object.defineProperty(this, "_url", {value: url});
   }
   async url() {
@@ -95,9 +96,13 @@ export function NoFileAttachments(name) {
 export default function FileAttachments(resolve) {
   return Object.assign(
     name => {
-      const url = resolve(name += ""); // Returns a Promise, string, or null.
-      if (url == null) throw new Error(`File not found: ${name}`);
-      return new FileAttachment(url, name);
+      const result = resolve(name += "");
+      if (result == null) throw new Error(`File not found: ${name}`);
+      if (typeof result === "object" && "url" in result) {
+        const {url, mimeType} = result;
+        return new FileAttachment(url, name, mimeType);
+      }
+      return new FileAttachment(result, name);
     },
     {prototype: FileAttachment.prototype} // instanceof
   );