Add README for avaje-jex-file-upload module

Added documentation for file upload module, including installation, usage, and configuration options.

Author: SentryMan

avaje-jex-file-upload/README.md

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+# File Uploads
+
+[![Maven Central](https://img.shields.io/maven-central/v/io.avaje/avaje-jex-file-upload.svg?label=Maven%20Central)](https://mvnrepository.com/artifact/io.avaje/avaje-jex-file-upload)
+[![javadoc](https://javadoc.io/badge2/io.avaje/avaje-jex-file-upload/javadoc.svg?color=purple)](https://javadoc.io/doc/io.avaje/avaje-jex-file-upload)
+
+Module for handling file uploads via multipart form data.
+
+It provides a `FileUploadPlugin` to configure file upload behavior and a `FileUploadService` to access uploaded files in your route handlers.
+
+## Installation
+
+Add the file upload dependency to your project:
+```xml
+<dependency>
+  <groupId>io.avaje</groupId>
+  <artifactId>avaje-jex-file-upload</artifactId>
+  <version>${avaje.jex.version}</version>
+</dependency>
+```
+
+## Basic Usage
+
+### Default Configuration
+```java
+// With default configuration
+Jex app = Jex.create().plugin(FileUploadPlugin.create());
+```
+
+### Custom Configuration
+```java
+// With custom configuration
+FileUploadPlugin uploadPlugin =
+    FileUploadPlugin.create(
+        config ->
+            config
+                .cacheDirectory("/tmp/uploads")
+                .maxFileSize(10, MB)
+                .maxRequestSize(50, MB)
+                .maxInMemoryFileSize(1, MB));
+
+Jex app = Jex.create().plugin(uploadPlugin);
+```
+
+## Accessing Uploaded Files
+
+Access uploaded files in your routes using the `FileUploadService`:
+```java
+app.post(
+    "/upload",
+    ctx -> {
+      var uploadService = ctx.attribute(FileUploadService.class);
+
+      // Get a single file
+      MultiPart file = uploadService.uploadedFile("avatar");
+
+      // Get multiple files with the same field name
+      List<MultiPart> files = uploadService.uploadedFiles("documents");
+
+      // Get all uploaded files
+      List<MultiPart> allFiles = uploadService.uploadedFiles();
+
+      // Get files grouped by field name
+      Map<String, List<MultiPart>> fileMap = uploadService.uploadedFileMap();
+    });
+```
+
+## Configuration Options
+
+| Method | Description |
+|--------|-------------|
+| `cacheDirectory("/path/to/dir")` | Sets the directory where uploaded files exceeding the in-memory size limit will be cached. Defaults to the system's temporary directory. |
+| `maxFileSize(size, unit)` | Sets the maximum allowed size for a single uploaded file. Use -1 for no limit. |
+| `maxRequestSize(size, unit)` | Sets the maximum total size for a multipart request, including all files and form data. Use -1 for no limit. |
+| `maxInMemoryFileSize(size, unit)` | Sets the maximum size a file can be before it is written to disk. Files smaller than this are kept in memory. Use 0 to write all files to disk. |
+
+### File Size Units
+
+Available units: `BYTES`, `KB`, `MB`, `GB`