Money Manager EX allows adding every file as an attachment for one of its elements (like transactions, accounts, assets, etc..)

All attachments are copied into a user-defined folder and not simply linked to the original file: in this way, they will remain accessible also if the original files are removed and it’s possible to backup all of them by simply copying the entire folder.

Bring Your Own Cloud could synchronize these attachments and database files across all your devices,

Here you go a typical folder tree (assume you choose %DATABASE%, with mmex_attachment.mmb)

├── MMEX_mmex_attachment_Attachments
│   ├── Transaction
│   │   ├── Transaction_1_Attach1.png
│   │   ├── Transaction_1_Attach2.png
│   │   ├── Transaction_2_Attach1.png
│   │   ├── Transaction_2_Attach2.png
│   │   └── readme.txt
│   └── readme.txt
└── mmex_attachment.mmb