diff --git a/internal/workflow/step_transition.go b/internal/workflow/step_transition.go
index fbca8bd3..be8a5356 100644
--- a/internal/workflow/step_transition.go
+++ b/internal/workflow/step_transition.go
@@ -33,6 +33,14 @@ type StepTransitionService struct {
 var ErrInvalidStepTransition = errors.New("invalid step transition")
 var errTransitionForbidden = errors.New("forbidden")
 
+var screenshotAllowedMediaTypes = map[string]bool{
+	"image/png":  true,
+	"image/jpeg": true,
+	"image/jpg":  true,
+	"image/gif":  true,
+	"image/webp": true,
+}
+
 // WorkflowDefinitionServiceInterface defines the interface for workflow definition operations
 type WorkflowDefinitionServiceInterface interface {
 	GetByID(id *uuid.UUID) (*workflows.WorkflowDefinition, error)
@@ -288,8 +296,20 @@ func (s *StepTransitionService) validateTransition(currentStatus, newStatus stri
 }
 
 // validateEvidenceRequirements validates that all required evidence has been submitted
+// and that file media types are compatible with their declared evidence type.
 func (s *StepTransitionService) validateEvidenceRequirements(stepDef *workflows.WorkflowStepDefinition, submittedEvidence []EvidenceSubmission) error {
 
+	// Validate file type compatibility per submission.
+	for _, evidence := range submittedEvidence {
+		if evidence.EvidenceType == "screenshot" && evidence.MediaType != "" {
+			// Normalize: lowercase and strip parameters (e.g. "Image/PNG; charset=binary" → "image/png")
+			normalized := strings.ToLower(strings.TrimSpace(strings.SplitN(evidence.MediaType, ";", 2)[0]))
+			if !screenshotAllowedMediaTypes[normalized] {
+				return fmt.Errorf("evidence type 'screenshot' requires an image file (png/jpg/jpeg/gif/webp), got %q", evidence.MediaType)
+			}
+		}
+	}
+
 	// Build a map of submitted evidence types
 	submittedTypes := make(map[string]int)
 	for _, evidence := range submittedEvidence {
diff --git a/internal/workflow/step_transition_test.go b/internal/workflow/step_transition_test.go
index 05c00b77..c7da08d2 100644
--- a/internal/workflow/step_transition_test.go
+++ b/internal/workflow/step_transition_test.go
@@ -271,6 +271,81 @@ func TestTransitionStepStatus_UnexpectedPermissionLookupErrorBubbles(t *testing.
 	mockRole.AssertExpectations(t)
 }
 
+// BCH-1150: screenshot evidence should reject non-image file types.
+// Observed: validateEvidenceRequirements accepts PDF/DOC for screenshot type.
+// Expected: returns error when media type is not an image for screenshot evidence.
+func TestValidateEvidenceRequirements_ScreenshotRejectsNonImageMediaType(t *testing.T) {
+	svc := NewStepTransitionService(nil, nil, nil, nil, nil, nil, nil, nil, nil, nil)
+
+	stepDef := &workflows.WorkflowStepDefinition{
+		EvidenceRequired: []workflows.EvidenceRequirement{
+			{Type: "screenshot", Required: true},
+		},
+	}
+
+	err := svc.validateEvidenceRequirements(stepDef, []EvidenceSubmission{
+		{EvidenceType: "screenshot", MediaType: "application/pdf"},
+	})
+
+	require.Error(t, err)
+	assert.Contains(t, err.Error(), "screenshot")
+}
+
+// BCH-1150: screenshot evidence should accept image file types.
+func TestValidateEvidenceRequirements_ScreenshotAcceptsImageMediaType(t *testing.T) {
+	svc := NewStepTransitionService(nil, nil, nil, nil, nil, nil, nil, nil, nil, nil)
+
+	stepDef := &workflows.WorkflowStepDefinition{
+		EvidenceRequired: []workflows.EvidenceRequirement{
+			{Type: "screenshot", Required: true},
+		},
+	}
+
+	for _, imageType := range []string{"image/png", "image/jpeg", "image/jpg", "image/gif", "image/webp"} {
+		err := svc.validateEvidenceRequirements(stepDef, []EvidenceSubmission{
+			{EvidenceType: "screenshot", MediaType: imageType},
+		})
+		assert.NoError(t, err, "expected %s to be accepted for screenshot", imageType)
+	}
+}
+
+// BCH-1150: media type matching must be case-insensitive and strip parameters so that
+// values like "Image/PNG" or "image/png; charset=binary" are accepted for screenshot evidence.
+func TestValidateEvidenceRequirements_ScreenshotAcceptsNormalizedMediaTypes(t *testing.T) {
+	svc := NewStepTransitionService(nil, nil, nil, nil, nil, nil, nil, nil, nil, nil)
+
+	stepDef := &workflows.WorkflowStepDefinition{
+		EvidenceRequired: []workflows.EvidenceRequirement{
+			{Type: "screenshot", Required: true},
+		},
+	}
+
+	for _, imageType := range []string{"Image/PNG", "IMAGE/JPEG", "image/png; charset=binary"} {
+		err := svc.validateEvidenceRequirements(stepDef, []EvidenceSubmission{
+			{EvidenceType: "screenshot", MediaType: imageType},
+		})
+		assert.NoError(t, err, "expected normalized %q to be accepted for screenshot", imageType)
+	}
+}
+
+// BCH-1150: document evidence should accept a broader set of media types including PDF and images.
+func TestValidateEvidenceRequirements_DocumentAcceptsBroadMediaTypes(t *testing.T) {
+	svc := NewStepTransitionService(nil, nil, nil, nil, nil, nil, nil, nil, nil, nil)
+
+	stepDef := &workflows.WorkflowStepDefinition{
+		EvidenceRequired: []workflows.EvidenceRequirement{
+			{Type: "document", Required: true},
+		},
+	}
+
+	for _, mediaType := range []string{"application/pdf", "application/msword", "image/png"} {
+		err := svc.validateEvidenceRequirements(stepDef, []EvidenceSubmission{
+			{EvidenceType: "document", MediaType: mediaType},
+		})
+		assert.NoError(t, err, "expected %s to be accepted for document", mediaType)
+	}
+}
+
 type mockStepAssignmentService struct {
 	called bool
 }