diff --git a/app/common/Features.ts b/app/common/Features.ts
index 8a5f10b7..a1af60a5 100644
--- a/app/common/Features.ts
+++ b/app/common/Features.ts
@@ -47,6 +47,8 @@ export interface Features {
                                  // Actual max for a document may be higher.
   baseMaxApiUnitsPerDocumentPerDay?: number;  // Similar for api calls.
   baseMaxDataSizePerDocument?: number;  // Similar maximum for number of bytes of 'normal' data in a document
+  baseMaxAttachmentsBytesPerDocument?: number;  // Similar maximum for total number of bytes used
+                                                // for attached files in a document
 
   gracePeriodDays?: number;  // Duration of the grace period in days, before entering delete-only mode
 }
diff --git a/app/common/UserAPI.ts b/app/common/UserAPI.ts
index 680b6d35..9d392442 100644
--- a/app/common/UserAPI.ts
+++ b/app/common/UserAPI.ts
@@ -394,6 +394,9 @@ export interface DocAPI {
    * @param title Name of the spreadsheet that will be created (should use a Grist document's title)
    */
   sendToDrive(code: string, title: string): Promise<{url: string}>;
+  // Upload a single attachment and return the resulting metadata row ID.
+  // The arguments are passed to FormData.append.
+  uploadAttachment(value: string | Blob, filename?: string): Promise<number>;
 }
 
 // Operations that are supported by a doc worker.
@@ -869,6 +872,20 @@ export class DocAPIImpl extends BaseAPI implements DocAPI {
     return this.requestJson(url.href);
   }
 
+  public async uploadAttachment(value: string | Blob, filename?: string): Promise<number> {
+    const formData = this.newFormData();
+    formData.append('upload', value, filename);
+    const response = await this.requestAxios(`${this._url}/attachments`, {
+      method: 'POST',
+      data: formData,
+      // On browser, it is important not to set Content-Type so that the browser takes care
+      // of setting HTTP headers appropriately.  Outside browser, requestAxios has logic
+      // for setting the HTTP headers.
+      headers: {...this.defaultHeadersWithoutContentType()},
+    });
+    return response.data[0];
+  }
+
   private _getRecords(tableId: string, endpoint: 'data' | 'records', options?: GetRowsParams): Promise<any> {
     const url = new URL(`${this._url}/tables/${tableId}/${endpoint}`);
     if (options?.filters) {
diff --git a/app/gen-server/entity/Product.ts b/app/gen-server/entity/Product.ts
index 6d364757..6c803943 100644
--- a/app/gen-server/entity/Product.ts
+++ b/app/gen-server/entity/Product.ts
@@ -39,6 +39,7 @@ export const teamFreeFeatures: Features = {
   baseMaxRowsPerDocument: 5000,
   baseMaxApiUnitsPerDocumentPerDay: 5000,
   baseMaxDataSizePerDocument: 5000 * 2 * 1024,  // 2KB per row
+  baseMaxAttachmentsBytesPerDocument: 1 * 1024 * 1024 * 1024,  // 1GB
   gracePeriodDays: 14,
 };
 
diff --git a/app/server/lib/ActiveDoc.ts b/app/server/lib/ActiveDoc.ts
index fd4fcdd5..07859c8e 100644
--- a/app/server/lib/ActiveDoc.ts
+++ b/app/server/lib/ActiveDoc.ts
@@ -96,6 +96,7 @@ import {findOrAddAllEnvelope, Sharing} from './Sharing';
 import cloneDeep = require('lodash/cloneDeep');
 import flatten = require('lodash/flatten');
 import remove = require('lodash/remove');
+import sum = require('lodash/sum');
 import without = require('lodash/without');
 import zipObject = require('lodash/zipObject');
 
@@ -650,6 +651,7 @@ export class ActiveDoc extends EventEmitter {
     const userId = getDocSessionUserId(docSession);
     const upload: UploadInfo = globalUploadSet.getUploadInfo(uploadId, this.makeAccessId(userId));
     try {
+      await this._checkDocAttachmentsLimit(upload);
       const userActions: UserAction[] = await Promise.all(
         upload.files.map(file => this._prepAttachment(docSession, file)));
       const result = await this.applyUserActions(docSession, userActions);
@@ -1307,11 +1309,12 @@ export class ActiveDoc extends EventEmitter {
    * 'soft deleting' them so that they get cleaned up automatically from _gristsys_Files after enough time has passed.
    * Set timeDeleted to null on used attachments that were previously soft deleted,
    * so that undo can 'undelete' attachments.
+   * Returns true if any changes were made, i.e. some row(s) of _grist_Attachments were updated.
    */
   public async updateUsedAttachments() {
     const changes = await this.docStorage.scanAttachmentsForUsageChanges();
     if (!changes.length) {
-      return;
+      return false;
     }
     const rowIds = changes.map(r => r.id);
     const now = Date.now() / 1000;
@@ -1319,6 +1322,7 @@ export class ActiveDoc extends EventEmitter {
     const action: BulkUpdateRecord = ["BulkUpdateRecord", "_grist_Attachments", rowIds, {timeDeleted}];
     // Don't use applyUserActions which may block the update action in delete-only mode
     await this._applyUserActions(makeExceptionalDocSession('system'), [action]);
+    return true;
   }
 
   /**
@@ -1886,6 +1890,40 @@ export class ActiveDoc extends EventEmitter {
       },
     });
   }
+
+  /**
+   * Throw an error if the provided upload would exceed the total attachment filesize limit for this document.
+   */
+  private async _checkDocAttachmentsLimit(upload: UploadInfo) {
+    const maxSize = this._productFeatures?.baseMaxAttachmentsBytesPerDocument;
+    if (!maxSize) {
+      // This document has no limit, nothing to check.
+      return;
+    }
+
+    // Minor flaw: while we don't double-count existing duplicate files in the total size,
+    // we don't check here if any of the uploaded files already exist and could be left out of the calculation.
+    const totalAddedSize = sum(upload.files.map(f => f.size));
+
+    // Returns true if this upload won't bring the total over the limit.
+    const isOK = async () => (await this.docStorage.getTotalAttachmentFileSizes()) + totalAddedSize <= maxSize;
+
+    if (await isOK()) {
+      return;
+    }
+
+    // Looks like the limit is being exceeded.
+    // Check if any attachments are unused and can be soft-deleted to reduce the existing total size.
+    // We could do this from the beginning, but updateUsedAttachments is potentially expensive,
+    // so this optimises the common case of not exceeding the limit.
+    // updateUsedAttachments returns true if there were any changes. Otherwise there's no point checking isOK again.
+    if (await this.updateUsedAttachments() && await isOK()) {
+      return;
+    }
+
+    // TODO probably want a nicer error message here.
+    throw new Error("Exceeded attachments limit for document");
+  }
 }
 
 // Helper to initialize a sandbox action bundle with no values.
diff --git a/app/server/lib/DocStorage.ts b/app/server/lib/DocStorage.ts
index fbadee97..c30890af 100644
--- a/app/server/lib/DocStorage.ts
+++ b/app/server/lib/DocStorage.ts
@@ -376,6 +376,18 @@ export class DocStorage implements ISQLiteDB, OnDemandStorage {
         }
       },
 
+      async function(db: SQLiteDB): Promise<void> {
+        // Storage version 8.
+        // Migration to add an index to _grist_Attachments.fileIdent for fast joining against _gristsys_Files.ident.
+        const tables = await db.all(`SELECT * FROM sqlite_master WHERE type='table' AND name='_grist_Attachments'`);
+        if (!tables.length) {
+          // _grist_Attachments is created in the first Python migration.
+          // For the sake of migration tests on ancient documents, just skip this migration if the table doesn't exist.
+          return;
+        }
+        await db.exec(`CREATE INDEX _grist_Attachments_fileIdent ON _grist_Attachments(fileIdent)`);
+      },
+
     ]
   };
 
@@ -1228,6 +1240,31 @@ export class DocStorage implements ISQLiteDB, OnDemandStorage {
         });
   }
 
+  /**
+   * Returns the total number of bytes used for storing attachments that haven't been soft-deleted.
+   * May be stale if ActiveDoc.updateUsedAttachments isn't called first.
+   */
+  public async getTotalAttachmentFileSizes() {
+    const result = await this.get(`
+      SELECT SUM(len) AS total
+      FROM (
+        -- Using MAX(LENGTH()) instead of just LENGTH() is needed in the presence of GROUP BY
+        -- to make LENGTH() quickly read the stored length instead of actually reading the blob data.
+        -- We use LENGTH() in the first place instead of _grist_Attachments.fileSize because the latter can
+        -- be changed by users.
+        SELECT MAX(LENGTH(files.data)) AS len
+        FROM _gristsys_Files AS files
+          JOIN _grist_Attachments AS meta
+            ON meta.fileIdent = files.ident
+        WHERE meta.timeDeleted IS NULL  -- Don't count soft-deleted attachments
+        -- Duplicate attachments (i.e. identical file contents) are only stored once in _gristsys_Files
+        -- but can be duplicated in _grist_Attachments, so the GROUP BY prevents adding up duplicated sizes.
+        GROUP BY meta.fileIdent
+      )
+    `);
+    return result!.total as number;
+  }
+
   /**
    * Returns an array of objects where:
    *   - `id` is a row ID of _grist_Attachments
diff --git a/test/fixtures/docs/Hello.grist b/test/fixtures/docs/Hello.grist
index 652be345..86cf9a46 100644
Binary files a/test/fixtures/docs/Hello.grist and b/test/fixtures/docs/Hello.grist differ