diff --git a/README.md b/README.md
index 3ad728c..692fd36 100644
--- a/README.md
+++ b/README.md
@@ -33,6 +33,20 @@ We have JavaScript support for dynamic formsets as part of a form. For an exampl
 3. Render the formset's `empty_form` somewhere on the page inside an element with the attribute `data-formset-template-for-id="<The formset prefix>"`.
 4. To use buttons to add and remove formset items to and from the formset, use the attribute `data-formset-append-to-list-id="<The formset prefix>"` on the append button and `data-formset-remove-from-list-id="<The formset prefix>"` on the remove button.
 
+### URL Format Select Field
+
+You can use `data-url-format-selector-for` for `<select>` inputs meant to control the format of files specified in another input.
+
+```html
+<input type="url" id="my-url-input" />
+<select data-url-format-selector-for="my-url-input">
+  <option value="markdown">Markdown</option>
+  <option value="plaintext">Plaintext</option>
+</select>
+```
+
+This will change the select value when the URL in `#my-url-input` changes to something with a matching file extension.
+
 ### Stylesheets
 
 We use the CSS reset/normalizer from Tailwind named [preflight.css](core/static/css/preflight.css) which is fairly aggressive. Otherwise, all site styles are defined in [site.css](core/static/css/site.css) with global styles at the top and page styles at the bottom. We try to use BEM syntax where appropriate; for example, we have a `.button` class with modifiers like `.button--prominent` and `.button--danger`.
diff --git a/core/forms.py b/core/forms.py
index 368f499..10d76e5 100644
--- a/core/forms.py
+++ b/core/forms.py
@@ -59,10 +59,6 @@ class SchemaRefForm(ReferenceItemForm):
         help_text=f"Accepted formats: {', '.join(sorted(EXPLICITLY_SUPPORTED_FILE_EXTENSIONS))}"
     )
 
-
-    def __init__(self, *args, **kwargs):
-        super().__init__(*args, **kwargs)
-
     def clean_url(self):
         if not self.cleaned_data['url']:
             return None
@@ -103,6 +99,10 @@ class DocumentationItemForm(ReferenceItemForm):
         initial=''
     )
 
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        self.fields['format'].widget.attrs['data-url-format-selector-for'] = self['url'].id_for_label
+
 
 DocumentationItemFormsetFactory = forms.formset_factory(DocumentationItemForm, extra=0)
 
@@ -122,7 +122,8 @@ class SchemaForm(forms.Form):
         choices=[('', 'Other')] + list(DocumentationItem.DocumentationItemFormat.choices),
         required=False,
         label="README format",
-        help_text="Markdown and Plaintext READMEs are displayed on Schemas.Pub"
+        help_text="Markdown and Plaintext READMEs are displayed on Schemas.Pub",
+        widget=forms.Select(attrs={'data-url-format-selector-for': 'id_readme_url'})
     )
     license_url = forms.URLField(
         label="License URL",
@@ -132,6 +133,7 @@ class SchemaForm(forms.Form):
 
     def __init__(self, *args, schema = None, **kwargs):
         super().__init__(*args, **kwargs)
+
         if schema == None:
             self.additional_documentation_items_formset = DocumentationItemFormsetFactory(prefix="documentation_items", *args, **kwargs)
             SchemaRefFormsetFactory = forms.formset_factory(SchemaRefForm, extra=1)
diff --git a/core/static/js/site.js b/core/static/js/site.js
index 1fcf7d0..84f5f46 100644
--- a/core/static/js/site.js
+++ b/core/static/js/site.js
@@ -1,6 +1,11 @@
 (() => {
   // This needs to match the animation duration in site.css
   const MESSAGE_TIMEOUT_MS = 10 * 1000;
+  // These keys need to match the values of DocumentationItemFormat
+  const FORMAT_OPTION_VALUE_EXTENSIONS = {
+    markdown: ['md', 'markdown'],
+    plaintext: ['txt'],
+  };
 
   /**
    * @param {(...args: any) => any} fn
@@ -88,6 +93,68 @@
     mutationObserver.observe(formsetListElement, { childList: true });
   };
 
+  /**
+   * Pass a `select` element containing a list of file formats
+   * for a paired URL field. When the URL field is changed,
+   * the `select` will be updated to a matching format if possible.
+   *
+   * @param {HTMLSelectElement} formatSelectElement
+   */
+  const initializeFormatSelectElement = (formatSelectElement) => {
+    const triggerElementId = formatSelectElement.getAttribute(
+      'data-url-format-selector-for'
+    );
+    if (!triggerElementId) {
+      return;
+    }
+    const triggerElement = document.getElementById(triggerElementId);
+    // If the URL input already has a value (like when editing an existing entity), bail.
+    if (!(triggerElement instanceof HTMLInputElement) || triggerElement.value) {
+      return;
+    }
+    // Get the available formats and filter by the ones we know extensions for
+    /** @type {(keyof FORMAT_OPTION_VALUE_EXTENSIONS)[]} */
+    const availableFormats = Array.from(formatSelectElement.options)
+      .map(({ value }) => value)
+      .filter(
+        /** @type {(value: string) => value is keyof FORMAT_OPTION_VALUE_EXTENSIONS } */
+        (value) => value in FORMAT_OPTION_VALUE_EXTENSIONS
+      );
+    // When the input element changes, we'll try to match its extension.
+    // If there's a match, we'll select it in the format dropdown.
+    const handleTriggerElementInput = () => {
+      try {
+        const url = new URL(triggerElement.value);
+        const matchingFormat = availableFormats.find((value) =>
+          FORMAT_OPTION_VALUE_EXTENSIONS[value].find((extension) =>
+            url.pathname.toLowerCase().endsWith('.' + extension)
+          )
+        );
+        if (matchingFormat) {
+          formatSelectElement.value = matchingFormat;
+        }
+        // eslint-disable-next-line no-unused-vars
+      } catch (err) {
+        // Fine; don't mess with the select element
+      }
+    };
+    triggerElement.addEventListener('input', handleTriggerElementInput);
+    // If the user manually changes the format selection,
+    // stop trying to set it for them.
+    // 'change' events do *not* fire when we set the value programmatically
+    const handleFormatSelectElementChange = () => {
+      triggerElement.removeEventListener('input', handleTriggerElementInput);
+      formatSelectElement.removeEventListener(
+        'change',
+        handleFormatSelectElementChange
+      );
+    };
+    formatSelectElement.addEventListener(
+      'change',
+      handleFormatSelectElementChange
+    );
+  };
+
   document.addEventListener('DOMContentLoaded', () => {
     Array.from(document.querySelectorAll('.js-autosubmit-input'))
       // If the input isn't in a form, there's nothing to submit
@@ -175,9 +242,28 @@
           currentFormItemCount.toString()
         );
         formsetListElement.insertAdjacentHTML('beforeend', nextFormItemHtml);
+        const formsetItems = formsetListElement.querySelectorAll('.formset');
+        Array.from(
+          formsetItems[formsetItems.length - 1].querySelectorAll(
+            '[data-url-format-selector-for]'
+          )
+        )
+          .filter((element) => element instanceof HTMLSelectElement)
+          .forEach((formatSelectElement) => {
+            initializeFormatSelectElement(formatSelectElement);
+          });
       });
     });
 
+    Array.from(document.querySelectorAll('[data-url-format-selector-for]'))
+      .filter(
+        (formatSelectElement) =>
+          formatSelectElement instanceof HTMLSelectElement
+      )
+      .forEach((formatSelectElement) =>
+        initializeFormatSelectElement(formatSelectElement)
+      );
+
     setTimeout(() => {
       Array.from(document.querySelectorAll('.messages .message')).forEach(
         (element) => {