fix: Content-Disposition header format to comply with RFC 2183

Fixes #8914

The Content-Disposition header was set to `filename="name.ext"` which
doesn't match RFC 2183 specification. This caused third-party libraries
(e.g., Go's mime.ParseMediaType) to fail parsing the header.

Changed to `inline; filename="name.ext"` format which properly includes
the disposition-type as required by RFC 2183.

server.py

@@ -511,7 +511,7 @@ class PromptServer():
                             buffer.seek(0)
 
                             return web.Response(body=buffer.read(), content_type=f'image/{image_format}',
-                                                headers={"Content-Disposition": f"filename=\"{filename}\""})
+                                                headers={"Content-Disposition": f"inline; filename=\"{filename}\""})
 
                     if 'channel' not in request.rel_url.query:
                         channel = 'rgba'
@@ -531,7 +531,7 @@ class PromptServer():
                             buffer.seek(0)
 
                             return web.Response(body=buffer.read(), content_type='image/png',
-                                                headers={"Content-Disposition": f"filename=\"{filename}\""})
+                                                headers={"Content-Disposition": f"inline; filename=\"{filename}\""})
 
                     elif channel == 'a':
                         with Image.open(file) as img:
@@ -548,7 +548,7 @@ class PromptServer():
                             alpha_buffer.seek(0)
 
                             return web.Response(body=alpha_buffer.read(), content_type='image/png',
-                                                headers={"Content-Disposition": f"filename=\"{filename}\""})
+                                                headers={"Content-Disposition": f"inline; filename=\"{filename}\""})
                     else:
                         # Get content type from mimetype, defaulting to 'application/octet-stream'
                         content_type = mimetypes.guess_type(filename)[0] or 'application/octet-stream'
@@ -560,7 +560,7 @@ class PromptServer():
                         return web.FileResponse(
                             file,
                             headers={
-                                "Content-Disposition": f"filename=\"{filename}\"",
+                                "Content-Disposition": f"inline; filename=\"{filename}\"",
                                 "Content-Type": content_type
                             }
                         )

tests-unit/server_test/test_content_disposition.py

@@ -0,0 +1,75 @@
+"""Tests for Content-Disposition header format (RFC 2183 compliance)
+
+Relates to issue #8914: Content-Disposition Header not matching RFC2183 rules
+"""
+
+import pytest
+import re
+
+
+class TestContentDispositionHeader:
+    """Test Content-Disposition header format compliance with RFC 2183."""
+
+    def test_rfc2183_format_with_inline(self):
+        """Verify inline disposition type format matches RFC 2183."""
+        filename = "test_image.png"
+        header = f'inline; filename="{filename}"'
+
+        # RFC 2183 requires disposition-type followed by parameters
+        # Format: disposition-type ";" disposition-parm
+        pattern = r'^(inline|attachment);\s*filename="[^"]*"$'
+        assert re.match(pattern, header), f"Header '{header}' does not match RFC 2183 format"
+
+    def test_rfc2183_format_with_attachment(self):
+        """Verify attachment disposition type format matches RFC 2183."""
+        filename = "download.mp4"
+        header = f'attachment; filename="{filename}"'
+
+        pattern = r'^(inline|attachment);\s*filename="[^"]*"$'
+        assert re.match(pattern, header), f"Header '{header}' does not match RFC 2183 format"
+
+    def test_invalid_format_missing_disposition_type(self):
+        """Verify that format without disposition type is invalid."""
+        filename = "test.jpg"
+        invalid_header = f'filename="{filename}"'
+
+        pattern = r'^(inline|attachment);\s*filename="[^"]*"$'
+        assert not re.match(pattern, invalid_header), \
+            "Header without disposition type should not match RFC 2183 format"
+
+    @pytest.mark.parametrize("filename", [
+        "image.png",
+        "video.mp4",
+        "file with spaces.jpg",
+        "special_chars-123.webp",
+    ])
+    def test_various_filenames(self, filename):
+        """Test RFC 2183 format with various filename patterns."""
+        header = f'inline; filename="{filename}"'
+
+        # Should have disposition type before filename
+        assert header.startswith("inline; ") or header.startswith("attachment; ")
+        assert f'filename="{filename}"' in header
+
+
+class TestContentDispositionParsing:
+    """Test that Content-Disposition headers can be parsed by standard libraries."""
+
+    def test_parse_inline_disposition(self):
+        """Test parsing inline disposition header."""
+        header = 'inline; filename="test.png"'
+
+        # Simple parsing test - split by semicolon
+        parts = [p.strip() for p in header.split(';')]
+        assert parts[0] in ('inline', 'attachment')
+        assert 'filename=' in parts[1]
+
+    def test_extract_filename(self):
+        """Test extracting filename from header."""
+        filename = "my_image.jpg"
+        header = f'inline; filename="{filename}"'
+
+        # Extract filename using regex
+        match = re.search(r'filename="([^"]*)"', header)
+        assert match is not None
+        assert match.group(1) == filename