feat(conversion): refactor EML parser to use Simple Java Mail library and add MSG support (#5427)

# Description of Changes


Note on Simple Java Mail:
- SJM contains Angus/Jakarta Mail in it.
- SJM is a very thin layer on Angus Mail; see here:
https://github.com/bbottema/simple-java-mail
- SJM gives high level methods to more reliably parse in email via Angus
Mail, but also contains lots of other interesting features.
- SJM is Apache 2 licensed

This pull request updates the email processing utilities to add support
for parsing and validating Outlook MSG files, refactors the
`EmlProcessingUtils` utility class to use instance methods and improved
resource management, and enhances the handling and styling of generated
email HTML. The changes also introduce external CSS resource loading
with a fallback mechanism, and update dependencies to support MSG file
parsing.

**MSG file support and validation:**
- Added `simple-java-mail` and `outlook-module` dependencies to enable
EML and MSG file parsing, and updated validation logic to recognize and
accept MSG files by checking their magic bytes.
(`app/common/build.gradle`, `EmlProcessingUtils.java`)
**Refactoring and modernization of `EmlProcessingUtils`:**
- Converted static methods and fields in `EmlProcessingUtils` to
instance methods/fields, improving testability and future extensibility.
(`EmlProcessingUtils.java`)

**Enhanced HTML/CSS styling for email rendering:**
- Updated HTML generation to use consistent formatting and improved
style variable usage, and refactored CSS injection to load from an
external resource (`email-pdf-styles.css`) with a synchronized cache and
a minimal fallback if the resource is missing.
(`EmlProcessingUtils.java`)
**Attachment and content rendering improvements:**
- Improved the formatting of meta-information (e.g., CC, BCC, Date) and
attachment sections in generated email HTML, and ensured more robust
handling of empty or missing content. (`EmlProcessingUtils.java`)

**General code cleanup and logging:**
- Added SLF4J logging for error handling when loading CSS resources, and
cleaned up imports and method signatures for clarity and
maintainability. (`EmlProcessingUtils.java`)


<img width="367" height="991" alt="image"
src="https://github.com/user-attachments/assets/0cfb959c-da92-4cff-9e52-ff4ab7fa806e"
/>


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- What was changed
- Why the change was made
- Any challenges encountered

Closes #(issue_number)
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---

## Checklist

### General

- [X] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [X] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md)
(if applicable)
- [X] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [X] I have performed a self-review of my own code
- [X] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### Translations (if applicable)

- [ ] I ran
[`scripts/counter_translation.py`](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/docs/counter_translation.md)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [X] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md#6-testing)
for more details.

---------

Signed-off-by: Balázs Szücs <bszucs1209@gmail.com>

frontend/src/core/components/tools/convert/ConvertSettings.tsx

@@ -338,8 +338,8 @@ const ConvertSettings = ({
         </>
       ) : null}
 
-      {/* Email to PDF options */}
-      {parameters.fromExtension === 'eml' && parameters.toExtension === 'pdf' && (
+      {/* Email to PDF options (EML and MSG formats) */}
+      {(parameters.fromExtension === 'eml' || parameters.fromExtension === 'msg') && parameters.toExtension === 'pdf' && (
         <>
           <Divider />
           <ConvertFromEmailSettings

frontend/src/core/constants/convertConstants.ts

@@ -96,6 +96,7 @@ export const FROM_FORMAT_OPTIONS = [
   { value: 'txt', label: 'TXT', group: 'Text' },
   { value: 'rtf', label: 'RTF', group: 'Text' },
   { value: 'eml', label: 'EML', group: 'Email' },
+  { value: 'msg', label: 'MSG (Outlook)', group: 'Email' },
   { value: 'epub', label: 'EPUB', group: 'eBook' },
   { value: 'mobi', label: 'MOBI', group: 'eBook' },
   { value: 'azw3', label: 'AZW3', group: 'eBook' },
@@ -140,6 +141,7 @@ export const CONVERSION_MATRIX: Record<string, string[]> = {
   'md': ['pdf'],
   'txt': ['pdf'], 'rtf': ['pdf'],
   'eml': ['pdf'],
+  'msg': ['pdf'],
   'cbr': ['pdf'],
   'epub': ['pdf'], 'mobi': ['pdf'], 'azw3': ['pdf'], 'fb2': ['pdf']
 };
@@ -171,6 +173,7 @@ export const EXTENSION_TO_ENDPOINT: Record<string, Record<string, string>> = {
   'txt': { 'pdf': 'file-to-pdf' }, 'rtf': { 'pdf': 'file-to-pdf' },
   'cbr': { 'pdf': 'cbr-to-pdf' },
   'eml': { 'pdf': 'eml-to-pdf' },
+  'msg': { 'pdf': 'eml-to-pdf' }, // MSG uses same endpoint as EML
   'epub': { 'pdf': 'ebook-to-pdf' }, 'mobi': { 'pdf': 'ebook-to-pdf' }, 'azw3': { 'pdf': 'ebook-to-pdf' }, 'fb2': { 'pdf': 'ebook-to-pdf' }
 };
 

frontend/src/core/constants/convertSupportedFornats.ts

@@ -11,7 +11,7 @@ export const CONVERT_SUPPORTED_FORMATS = [
   // StarOffice
   'sda', 'sdc', 'sdd', 'sdw', 'stc', 'std', 'sti', 'stw', 'sxd', 'sxg', 'sxi', 'sxw',
   // Email formats
-  'eml',
+  'eml', 'msg',
   // Ebook formats
   'epub', 'mobi', 'azw3', 'fb2',
   // Archive formats

frontend/src/core/hooks/tools/convert/useConvertOperation.ts

@@ -66,7 +66,7 @@ export const buildConvertFormData = (parameters: ConvertParameters, selectedFile
     formData.append("autoRotate", imageOptions.autoRotate.toString());
   } else if ((fromExtension === 'html' || fromExtension === 'zip') && toExtension === 'pdf') {
     formData.append("zoom", htmlOptions.zoomLevel.toString());
-  } else if (fromExtension === 'eml' && toExtension === 'pdf') {
+  } else if ((fromExtension === 'eml' || fromExtension === 'msg') && toExtension === 'pdf') {
     formData.append("includeAttachments", emailOptions.includeAttachments.toString());
     formData.append("maxAttachmentSizeMB", emailOptions.maxAttachmentSizeMB.toString());
     formData.append("downloadHtml", emailOptions.downloadHtml.toString());

frontend/src/core/tests/helpers/conversionEndpointDiscovery.ts

@@ -120,6 +120,13 @@ const ALL_CONVERSION_ENDPOINTS: ConversionEndpoint[] = [
     toFormat: 'pdf',
     description: 'Convert email (EML) to PDF',
     apiPath: '/api/v1/convert/eml/pdf'
+  },
+  {
+    endpoint: 'eml-to-pdf', // MSG uses same endpoint as EML
+    fromFormat: 'msg',
+    toFormat: 'pdf',
+    description: 'Convert Outlook email (MSG) to PDF',
+    apiPath: '/api/v1/convert/eml/pdf'
   }
 ];
 

frontend/src/core/utils/convertUtils.test.ts

@@ -76,8 +76,9 @@ describe('convertUtils', () => {
       expect(getEndpointName('txt', 'pdf')).toBe('file-to-pdf');
       expect(getEndpointName('rtf', 'pdf')).toBe('file-to-pdf');
       
-      // Email to PDF
+      // Email to PDF (EML and MSG)
       expect(getEndpointName('eml', 'pdf')).toBe('eml-to-pdf');
+      expect(getEndpointName('msg', 'pdf')).toBe('eml-to-pdf');
     });
 
     test('should return empty string for unsupported conversions', () => {
@@ -158,8 +159,9 @@ describe('convertUtils', () => {
       expect(getEndpointUrl('txt', 'pdf')).toBe('/api/v1/convert/file/pdf');
       expect(getEndpointUrl('rtf', 'pdf')).toBe('/api/v1/convert/file/pdf');
       
-      // Email to PDF
+      // Email to PDF (EML and MSG)
       expect(getEndpointUrl('eml', 'pdf')).toBe('/api/v1/convert/eml/pdf');
+      expect(getEndpointUrl('msg', 'pdf')).toBe('/api/v1/convert/eml/pdf');
     });
 
     test('should return empty string for unsupported conversions', () => {
@@ -240,8 +242,9 @@ describe('convertUtils', () => {
       expect(isConversionSupported('txt', 'pdf')).toBe(true);
       expect(isConversionSupported('rtf', 'pdf')).toBe(true);
       
-      // Email to PDF
+      // Email to PDF (EML and MSG)
       expect(isConversionSupported('eml', 'pdf')).toBe(true);
+      expect(isConversionSupported('msg', 'pdf')).toBe(true);
     });
 
     test('should return false for unsupported conversions', () => {