Uploaded a new version of a document using Content Server API but cannot open the file

I have managed to get the uploading of new versions of word files version to OT working using /v2/nodes/{id}/versions. When trying to open the new version we are receiving following messages:

"Word found unreadable content in <filename>. Do you want to recover the content...." and if clicking "Yes" I am getting another dialog saying "Word experienced an error trying to open the file...". 

So, uploading works but something makes the file corrupt and I need help get this right.

I've tried a couple of ways on our side and the body sent is either of these two for my tries (neither correspond to how the content should look like but it may also be problem printing the data correct to the log itself):

First example:

----WebKitFormBoundary7MA4YWxkTrZu0gW
Content-Disposition: form-data; name="file"; filename="file1.docx"
Content-Type: application/vnd.openxmlformats-officedocument.wordprocessingml.document

PK!ߤ�lZ [Content_Types].xml �(����n�0E�����Ub袪*�>�-R�{V��Ǽ��QU�
l"%3��3Vƃ�ښl �w%�=���^i7+���-d&�0�A�6�l4��L60#�Ò�S

---cutting---

)NN~�"word/webSettings.xmlPK-!�/��z;$word/fontTable.xmlPK-!|d�|Z&docProps/core.xmlPK-!�y��q� )docProps/app.xmlPK��+
----WebKitFormBoundary7MA4YWxkTrZu0gW--

Second example:

----WebKitFormBoundary7MA4YWxkTrZu0gW
Content-Disposition: form-data; name="file"; filename="file1.docx"
Content-Type: application/vnd.openxmlformats-officedocument.wordprocessingml.document

PK!0ýýý[Content_Types].xml ý(ýýný0EýýýýýUb`QUýEýýýL<!Výýýýýýýý
*DýDýgýgýDýpýýeýý5)ý'=ýlTfýýýýkýýýýýHQZ)ý@`ýýýýpýq"Rýýýý=rýýýu`ýý[ý­ýs'ý/1>ýýýyf 

---cutting---

ý#word/webSettings.xmlPK-!Gý`~ýýý$word/fontTable.xmlPK-!mýuýýý'docProps/core.xmlPK-!ýBtý3ýý*word/numbering.xmlPK-!ýýýGý3docProps/app.xmlPKMý7
----WebKitFormBoundary7MA4YWxkTrZu0gW--

Http header Content-Type = multipart/form-data;boundary=--WebKitFormBoundary7MA4YWxkTrZu0g

Not sure what additional information to add. Anyone can point to any reason why this may happen?