fix(form-fill): fix hardcoded response (#5755)

# Description of Changes

Closes: #5752

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## Checklist

### General

- [ ] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [ ] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md)
(if applicable)
- [ ] 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)
- [ ] I have performed a self-review of my own code
- [ ] 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)

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### Testing (if applicable)

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

frontend/src/proprietary/tools/formFill/formApi.ts

@@ -16,10 +16,7 @@ export async function fetchFormFieldsWithCoordinates(
 
   const response = await apiClient.post<FormField[]>(
     '/api/v1/form/fields-with-coordinates',
-    formData,
-    {
-      headers: { 'Content-Type': 'multipart/form-data' },
-    }
+    formData
   );
   return response.data;
 }
@@ -42,7 +39,6 @@ export async function fillFormFields(
   formData.append('flatten', String(flatten));
 
   const response = await apiClient.post('/api/v1/form/fill', formData, {
-    headers: { 'Content-Type': 'multipart/form-data' },
     responseType: 'blob',
   });
   return response.data;