diff --git a/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/PythonFastAPIServerCodegen.java b/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/PythonFastAPIServerCodegen.java
index b820960915d1..ea617b75581b 100644
--- a/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/PythonFastAPIServerCodegen.java
+++ b/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/PythonFastAPIServerCodegen.java
@@ -243,6 +243,7 @@ public OperationsMap postProcessOperationsWithModels(OperationsMap objs, List<Mo
         OperationMap operations = objs.getOperations();
         // Set will make sure that no duplicated items are used.
         Set<String> securityImports = new HashSet<>();
+        boolean hasFileFormParam = false;
         if (operations != null) {
             List<CodegenOperation> ops = operations.getOperation();
             for (final CodegenOperation operation : ops) {
@@ -254,14 +255,64 @@ public OperationsMap postProcessOperationsWithModels(OperationsMap objs, List<Mo
                 }
 
                 setBodyParamExampleFromContent(operation);
+                if (overrideFileFormParamTyping(operation)) {
+                    hasFileFormParam = true;
+                }
             }
         }
 
+        if (hasFileFormParam) {
+            addFastAPIUploadFileImport(objs);
+        }
+
         objs.put("securityImports", new ArrayList<>(securityImports));
 
         return objs;
     }
 
+    /**
+     * Overrides {@code x-py-typing} for binary multipart form parameters so that they
+     * are typed as FastAPI {@code UploadFile} instead of the client-side bytes/str union.
+     * FastAPI parses multipart {@code format: binary} fields into {@link UploadFile} instances;
+     * the default Pydantic-based union ({@code Union[StrictBytes, StrictStr, ...]}) rejects
+     * them with a 422 at request time.
+     *
+     * @param operation the operation whose parameters may need rewriting
+     * @return {@code true} if at least one parameter was rewritten
+     */
+    private boolean overrideFileFormParamTyping(CodegenOperation operation) {
+        boolean changed = false;
+        for (CodegenParameter param : operation.allParams) {
+            if (param.isFormParam && param.isFile) {
+                param.vendorExtensions.put("x-py-typing", param.required ? "UploadFile" : "Optional[UploadFile]");
+                changed = true;
+            }
+        }
+        for (CodegenParameter param : operation.formParams) {
+            if (param.isFile) {
+                param.vendorExtensions.put("x-py-typing", param.required ? "UploadFile" : "Optional[UploadFile]");
+            }
+        }
+        return changed;
+    }
+
+    private void addFastAPIUploadFileImport(OperationsMap objs) {
+        List<Map<String, String>> imports = objs.getImports();
+        if (imports == null) {
+            imports = new ArrayList<>();
+            objs.setImports(imports);
+        }
+        String importLine = "from fastapi import File, UploadFile";
+        for (Map<String, String> existing : imports) {
+            if (importLine.equals(existing.get("import"))) {
+                return;
+            }
+        }
+        Map<String, String> item = new HashMap<>();
+        item.put("import", importLine);
+        imports.add(item);
+    }
+
     private void setBodyParamExampleFromContent(CodegenOperation operation) {
         if (operation.bodyParam == null) {
             return;
diff --git a/modules/openapi-generator/src/main/resources/python-fastapi/endpoint_argument_definition.mustache b/modules/openapi-generator/src/main/resources/python-fastapi/endpoint_argument_definition.mustache
index 24e6da7bb28e..9925098fbb65 100644
--- a/modules/openapi-generator/src/main/resources/python-fastapi/endpoint_argument_definition.mustache
+++ b/modules/openapi-generator/src/main/resources/python-fastapi/endpoint_argument_definition.mustache
@@ -1 +1 @@
-{{#isPathParam}}{{baseName}}{{/isPathParam}}{{^isPathParam}}{{paramName}}{{/isPathParam}}: {{>param_type}} = {{#isPathParam}}Path{{/isPathParam}}{{#isHeaderParam}}Header{{/isHeaderParam}}{{#isFormParam}}Form{{/isFormParam}}{{#isQueryParam}}Query{{/isQueryParam}}{{#isCookieParam}}Cookie{{/isCookieParam}}{{#isBodyParam}}Body{{/isBodyParam}}({{&defaultValue}}{{^defaultValue}}{{#isPathParam}}...{{/isPathParam}}{{^isPathParam}}None{{/isPathParam}}{{/defaultValue}}, description="{{description}}"{{#isQueryParam}}, alias="{{baseName}}"{{/isQueryParam}}{{#isLong}}{{#minimum}}, ge={{.}}{{/minimum}}{{#maximum}}, le={{.}}{{/maximum}}{{/isLong}}{{#isInteger}}{{#minimum}}, ge={{.}}{{/minimum}}{{#maximum}}, le={{.}}{{/maximum}}{{/isInteger}}{{#vendorExtensions.x-regex}}, regex=r"{{.}}"{{/vendorExtensions.x-regex}}{{#minLength}}, min_length={{.}}{{/minLength}}{{#maxLength}}, max_length={{.}}{{/maxLength}}{{^isBodyParam}}{{#vendorExtensions.x-py-example}}, examples=[{{{.}}}]{{/vendorExtensions.x-py-example}}{{/isBodyParam}}{{#isBodyParam}}{{#vendorExtensions.x-py-fastapi-example}}, examples=[{{{.}}}]{{/vendorExtensions.x-py-fastapi-example}}{{/isBodyParam}})
\ No newline at end of file
+{{#isPathParam}}{{baseName}}{{/isPathParam}}{{^isPathParam}}{{paramName}}{{/isPathParam}}: {{>param_type}} = {{#isPathParam}}Path{{/isPathParam}}{{#isHeaderParam}}Header{{/isHeaderParam}}{{#isFormParam}}{{#isFile}}File{{/isFile}}{{^isFile}}Form{{/isFile}}{{/isFormParam}}{{#isQueryParam}}Query{{/isQueryParam}}{{#isCookieParam}}Cookie{{/isCookieParam}}{{#isBodyParam}}Body{{/isBodyParam}}({{&defaultValue}}{{^defaultValue}}{{#isPathParam}}...{{/isPathParam}}{{^isPathParam}}{{#isFile}}{{#required}}...{{/required}}{{^required}}None{{/required}}{{/isFile}}{{^isFile}}None{{/isFile}}{{/isPathParam}}{{/defaultValue}}, description="{{description}}"{{#isQueryParam}}, alias="{{baseName}}"{{/isQueryParam}}{{#isLong}}{{#minimum}}, ge={{.}}{{/minimum}}{{#maximum}}, le={{.}}{{/maximum}}{{/isLong}}{{#isInteger}}{{#minimum}}, ge={{.}}{{/minimum}}{{#maximum}}, le={{.}}{{/maximum}}{{/isInteger}}{{#vendorExtensions.x-regex}}, regex=r"{{.}}"{{/vendorExtensions.x-regex}}{{#minLength}}, min_length={{.}}{{/minLength}}{{#maxLength}}, max_length={{.}}{{/maxLength}}{{^isBodyParam}}{{#vendorExtensions.x-py-example}}, examples=[{{{.}}}]{{/vendorExtensions.x-py-example}}{{/isBodyParam}}{{#isBodyParam}}{{#vendorExtensions.x-py-fastapi-example}}, examples=[{{{.}}}]{{/vendorExtensions.x-py-fastapi-example}}{{/isBodyParam}})
\ No newline at end of file
diff --git a/modules/openapi-generator/src/test/java/org/openapitools/codegen/python/PythonFastAPIServerCodegenTest.java b/modules/openapi-generator/src/test/java/org/openapitools/codegen/python/PythonFastAPIServerCodegenTest.java
index 7cce9fc18705..ff381bacecbd 100644
--- a/modules/openapi-generator/src/test/java/org/openapitools/codegen/python/PythonFastAPIServerCodegenTest.java
+++ b/modules/openapi-generator/src/test/java/org/openapitools/codegen/python/PythonFastAPIServerCodegenTest.java
@@ -105,4 +105,35 @@ public void testToPythonExamplePrefersExampleOverExamples() {
 
         Assert.assertEquals(codegen.exposeToPythonExample(cp), "\"doggie\"");
     }
+
+    @Test(description = "binary multipart form fields are typed as FastAPI UploadFile")
+    public void testBinaryMultipartFieldUsesUploadFile() throws IOException {
+        final DefaultCodegen codegen = new PythonFastAPIServerCodegen();
+        final String outputPath = generateFiles(codegen, "src/test/resources/bugs/issue_20115.yaml");
+        final Path api = Paths.get(outputPath + "src/openapi_server/apis/default_api.py");
+        final Path baseApi = Paths.get(outputPath + "src/openapi_server/apis/default_api_base.py");
+
+        assertFileExists(api);
+        assertFileExists(baseApi);
+
+        // Required binary form field becomes `UploadFile = File(...)`
+        assertFileContains(api, "csv_file: UploadFile = File(..., description=\"The CSV file to upload\")");
+        // Optional binary form field becomes `Optional[UploadFile] = File(None, ...)`
+        assertFileContains(api, "image: Optional[UploadFile] = File(None, description=\"Optional image upload\")");
+
+        // Sibling non-binary form fields still use Form()
+        assertFileContains(api, "collection_name: Annotated[StrictStr, Field(description=\"Name of the collection\")] = Form(None, description=\"Name of the collection\")");
+
+        // The legacy client-side bytes union must not appear for the server signature
+        assertFileNotContains(api, "Union[StrictBytes, StrictStr, Tuple[StrictStr, StrictBytes]]");
+        assertFileNotContains(baseApi, "Union[StrictBytes, StrictStr, Tuple[StrictStr, StrictBytes]]");
+
+        // FastAPI File/UploadFile imports are emitted
+        assertFileContains(api, "from fastapi import File, UploadFile");
+        assertFileContains(baseApi, "from fastapi import File, UploadFile");
+
+        // Abstract base class uses UploadFile directly (no Annotated wrapper)
+        assertFileContains(baseApi, "csv_file: UploadFile,");
+        assertFileContains(baseApi, "image: Optional[UploadFile],");
+    }
 }
diff --git a/modules/openapi-generator/src/test/resources/bugs/issue_20115.yaml b/modules/openapi-generator/src/test/resources/bugs/issue_20115.yaml
new file mode 100644
index 000000000000..7f40d15cfa59
--- /dev/null
+++ b/modules/openapi-generator/src/test/resources/bugs/issue_20115.yaml
@@ -0,0 +1,44 @@
+openapi: 3.0.1
+info:
+  title: Issue 20115 reproducer
+  version: 1.0.0
+paths:
+  /upload:
+    post:
+      operationId: uploadCsv
+      requestBody:
+        required: true
+        content:
+          multipart/form-data:
+            schema:
+              type: object
+              properties:
+                csv_file:
+                  type: string
+                  format: binary
+                  description: The CSV file to upload
+                collection_name:
+                  type: string
+                  description: Name of the collection
+              required:
+                - csv_file
+                - collection_name
+      responses:
+        '200':
+          description: OK
+  /upload-optional:
+    post:
+      operationId: uploadOptional
+      requestBody:
+        content:
+          multipart/form-data:
+            schema:
+              type: object
+              properties:
+                image:
+                  type: string
+                  format: binary
+                  description: Optional image upload
+      responses:
+        '200':
+          description: OK
diff --git a/samples/server/petstore/python-fastapi/src/openapi_server/apis/pet_api.py b/samples/server/petstore/python-fastapi/src/openapi_server/apis/pet_api.py
index 416272bcd61d..660a8d99d437 100644
--- a/samples/server/petstore/python-fastapi/src/openapi_server/apis/pet_api.py
+++ b/samples/server/petstore/python-fastapi/src/openapi_server/apis/pet_api.py
@@ -28,6 +28,7 @@
 from typing_extensions import Annotated
 from openapi_server.models.api_response import ApiResponse
 from openapi_server.models.pet import Pet
+from fastapi import File, UploadFile
 from openapi_server.security_api import get_token_petstore_auth, get_token_api_key
 
 router = APIRouter()
@@ -207,7 +208,7 @@ async def delete_pet(
 async def upload_file(
     petId: Annotated[StrictInt, Field(description="ID of pet to update")] = Path(..., description="ID of pet to update"),
     additional_metadata: Annotated[Optional[StrictStr], Field(description="Additional data to pass to server")] = Form(None, description="Additional data to pass to server"),
-    file: Annotated[Optional[Union[StrictBytes, StrictStr, Tuple[StrictStr, StrictBytes]]], Field(description="file to upload")] = Form(None, description="file to upload"),
+    file: Optional[UploadFile] = File(None, description="file to upload"),
     token_petstore_auth: TokenModel = Security(
         get_token_petstore_auth, scopes=["write:pets", "read:pets"]
     ),
diff --git a/samples/server/petstore/python-fastapi/src/openapi_server/apis/pet_api_base.py b/samples/server/petstore/python-fastapi/src/openapi_server/apis/pet_api_base.py
index 0d1fb77e442d..4f8b292e3eec 100644
--- a/samples/server/petstore/python-fastapi/src/openapi_server/apis/pet_api_base.py
+++ b/samples/server/petstore/python-fastapi/src/openapi_server/apis/pet_api_base.py
@@ -7,6 +7,7 @@
 from typing_extensions import Annotated
 from openapi_server.models.api_response import ApiResponse
 from openapi_server.models.pet import Pet
+from fastapi import File, UploadFile
 from openapi_server.security_api import get_token_petstore_auth, get_token_api_key
 
 class BasePetApi:
@@ -78,7 +79,7 @@ async def upload_file(
         self,
         petId: Annotated[StrictInt, Field(description="ID of pet to update")],
         additional_metadata: Annotated[Optional[StrictStr], Field(description="Additional data to pass to server")],
-        file: Annotated[Optional[Union[StrictBytes, StrictStr, Tuple[StrictStr, StrictBytes]]], Field(description="file to upload")],
+        file: Optional[UploadFile],
     ) -> ApiResponse:
         """"""
         ...