mimeType for FileAttachment (#281)

* mimeType for FileAttachment

* coerce mimeType to string, if present

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
 

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
  );