diff --git a/documentation/api.md b/documentation/api.md
index cdd5039..c26f52f 100644
--- a/documentation/api.md
+++ b/documentation/api.md
@@ -220,10 +220,12 @@ Password protected content
 | 400 | Password too long (max 1024 chars) |
 | 400 | Proof-of-work required (when PoW enabled) |
 | 400 | Proof-of-work failed (invalid/expired challenge) |
+| 400 | Paste too small (below minimum size) |
 | 413 | Paste too large |
 | 429 | Duplicate content rate limit exceeded |
 
 **Size Limits:**
+- Minimum: disabled by default (`FLASKPASTE_MIN_SIZE`, e.g. 64 bytes for encryption enforcement)
 - Anonymous: 3 MiB (configurable via `FLASKPASTE_MAX_ANON`)
 - Authenticated: 50 MiB (configurable via `FLASKPASTE_MAX_AUTH`)
 
@@ -575,6 +577,50 @@ export FLASKPASTE_MIN_ENTROPY_SIZE=256 # Only check content >= this size (defaul
 
 ---
 
+### Binary Content Requirement
+
+FlaskPaste can require unrecognizable binary content (MIME-based encryption enforcement).
+
+**How it works:**
+- Content is checked for magic bytes (PNG, JPEG, PDF, ZIP, etc.)
+- Content is checked for valid UTF-8 text
+- Recognized formats are rejected with 400
+- Only application/octet-stream (unrecognizable binary) is allowed
+
+**Configuration:**
+```bash
+export FLASKPASTE_REQUIRE_BINARY=1  # Reject recognizable formats (0=disabled)
+```
+
+**Response (400 Bad Request):**
+```json
+{
+  "error": "Recognizable format not allowed",
+  "detected": "text/plain",
+  "hint": "Encrypt content before uploading (-e flag in fpaste)"
+}
+```
+
+**Detected formats:**
+- `text/plain` (valid UTF-8 text)
+- `image/png`, `image/jpeg`, `image/gif`, `image/webp`
+- `application/pdf`, `application/zip`, `application/gzip`
+
+**vs Entropy enforcement:**
+| Method | Detects | False positives |
+|--------|---------|-----------------|
+| Entropy | Random-looking data | Compressed files pass |
+| Binary | No magic bytes + invalid UTF-8 | Minimal |
+
+Use both together for maximum encryption enforcement:
+```bash
+export FLASKPASTE_REQUIRE_BINARY=1
+export FLASKPASTE_MIN_ENTROPY=6.0
+export FLASKPASTE_MIN_SIZE=64
+```
+
+---
+
 ## Proof-of-Work
 
 FlaskPaste includes an optional proof-of-work system to prevent automated spam.